Imaging device and electronic device

ABSTRACT

To provide an imaging device capable of obtaining high-quality imaging data. The imaging device includes a first circuit and a second circuit. The first circuit includes a photoelectric conversion element, a first transistor, a second transistor, a third transistor, a fourth transistor, a fifth transistor, a sixth transistor, a seventh transistor, a first capacitor, a second capacitor, and a third capacitor. The second circuit includes an eighth transistor. The imaging device can compensate variation in threshold voltage of an amplifier transistor included in the first circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to an imaging device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. One embodiment of thepresent invention relates to a process, a machine, manufacture, or acomposition of matter. Specifically, examples of the technical field ofone embodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a liquid crystaldisplay device, a light-emitting device, a lighting device, a powerstorage device, a storage device, an imaging device, a method fordriving any of them, and a method for manufacturing any of them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a storage device, adisplay device, an imaging device, or an electronic device includes asemiconductor device.

2. Description of the Related Art

As a semiconductor device in which pixels each provided with aphotosensor are arranged in a matrix, a complementary metal oxidesemiconductor (CMOS) image sensor is known. CMOS image sensors areprovided in many portable devices such as digital cameras or cellularphones as imaging elements.

Silicon is widely known as a semiconductor material applicable to atransistor generally included in a CMOS image sensor or the like. Asanother material, an oxide semiconductor has attracted attention.

For example, Patent Document 1 discloses that a transistor including anoxide semiconductor and having extremely low off-state current is usedin part of a pixel circuit and a transistor including a siliconsemiconductor with which a CMOS circuit can be formed is used in aperipheral circuit, so that an imaging device with high speed operationand low power consumption can be manufactured.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2011-119711

SUMMARY OF THE INVENTION

A CMOS image sensor includes an amplifier transistor for outputting datain each pixel. In order to obtain high-quality imaging data, electricalcharacteristics of the transistors in all the pixels are preferablyuninform. However, as miniaturization progresses, the degree ofdifficulty of a transistor manufacturing process increases, and it isdifficult to reduce variation in electrical characteristics.

Output data can be compensated by retaining data for compensatingvariation in electrical characteristics in a capacitor or the like.However, total imaging time becomes long if data is written to acapacitor by each imaging. In addition, the increase in powerconsumption becomes a problem.

Thus, an object of one embodiment of the present invention is to providean imaging device capable of obtaining high-quality imaging data.Another object of one embodiment of the present invention is to providean imaging device capable of compensating variation in threshold voltageof an amplifier transistor included in a pixel circuit. Another objectof one embodiment of the present invention is to provide a low-powerimaging device. Another object of one embodiment of the presentinvention is to provide an imaging device that is suitable forhigh-speed operation. Another object of one embodiment of the presentinvention is to provide an imaging device with high sensitivity. Anotherobject of one embodiment of the present invention is to provide animaging device with a wide dynamic range. Another object of oneembodiment of the present invention is to provide an imaging device withhigh resolution. Another object of one embodiment of the presentinvention is to provide an imaging device formed at low cost. Anotherobject of one embodiment of the present invention is to provide animaging device with high reliability. Another object of one embodimentof the present invention is to provide a novel imaging device or thelike. Another object of one embodiment of the present invention is toprovide a novel semiconductor device or the like.

The description of these objects does not disturb the existence of otherobjects. In one embodiment of the present invention, there is no need toachieve all the objects. Other objects will be apparent from and can bederived from the description of the specification, the drawings, theclaims, and the like. Other objects will be apparent from and can bederived from the description of the specification, the drawings, theclaims, and the like.

One embodiment of the present invention relates to an imaging devicecapable of compensating variation in threshold voltage of an amplifiertransistor included in a pixel circuit.

One embodiment of the present invention is an imaging device thatincludes a first circuit and a second circuit. The first circuitincludes a photoelectric conversion element, a first transistor, asecond transistor, a third transistor, a fourth transistor, a fifthtransistor, a sixth transistor, a seventh transistor, a first capacitor,a second capacitor, and a third capacitor. The second circuit includesan eighth transistor. One terminal of the photoelectric conversionelement is electrically connected to one of a source and a drain of thefirst transistor. The other of the source and the drain of the firsttransistor is electrically connected to one of a source and a drain ofthe second transistor. The other of the source and the drain of thefirst transistor is electrically connected to one terminal of the firstcapacitor. One of a source and a drain of the third transistor iselectrically connected to the other terminal of the first capacitor. Theother terminal of the first capacitor is electrically connected to oneterminal of the second capacitor. One of a source and a drain of thefourth transistor is electrically connected to the other terminal of thesecond capacitor. The other of the source and the drain of the fourthtransistor is electrically connected to one of a source and a drain ofthe fifth transistor. One of a source and a drain of the sixthtransistor is electrically connected to one of the source and the drainof the fifth transistor. One terminal of the third capacitor iselectrically connected to the other terminal of the second capacitor.The other terminal of the third capacitor is electrically connected tothe other of the source and the drain of the sixth transistor. A gate ofthe sixth transistor is electrically connected to the one terminal ofthe third capacitor. One of a source and a drain of the seventhtransistor is electrically connected to the other of the source and thedrain of the sixth transistor. The other of the source and the drain ofthe seventh transistor is electrically connected to one of a source anda drain of the eighth transistor. The photoelectric conversion elementcontains selenium in a photoelectric conversion layer.

The second circuit may further include a ninth transistor. One of asource and a drain of the ninth transistor may be electrically connectedto the other of the source and the drain of the eighth transistor. Agate of the ninth transistor may be electrically connected to a gate ofthe eighth transistor. The gate of the ninth transistor may beelectrically connected to the other of the source and the drain of theninth transistor.

The other of the source and the drain of the third transistor may beelectrically connected to the other terminal of the photoelectricconversion element.

The pixel circuit may further include a fourth capacitor. One terminalof the fourth capacitor may be electrically connected to one of thesource and the drain of the third transistor. The other terminal of thefourth capacitor may be electrically connected to the other of thesource and the drain of the fifth transistor.

One or all of the first to ninth transistors preferably include oxidesemiconductor in active layers, and the oxide semiconductor preferablyincludes In, Zn, and M (M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf).

According to one embodiment of the present invention, an imaging devicecapable of obtaining high-quality imaging data can be provided. Animaging device capable of compensating variation in electricalcharacteristics of an amplifier transistor included in a pixel circuitcan be provided. A low-power imaging device can be provided. An imagingdevice that is suitable for high-speed operation can be provided. Animaging device with high sensitivity can be provided. An imaging devicewith a wide dynamic range can be provided. An imaging device with highresolution can be provided. An imaging device formed at low cost can beprovided. An imaging device with high reliability can be provided. Anovel imaging device or the like can be provided. A novel semiconductordevice or the like can be provided.

The description of these effects does not disturb the existence of othereffects. In one embodiment of the present invention, there is no need toobtain all the effects. Other effects will be apparent from and can bederived from the description of the specification, the drawings, theclaims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates circuits of an imaging device;

FIG. 2 illustrates a circuit of an imaging device;

FIG. 3 is a timing chart illustrating compensation operation;

FIG. 4 illustrates compensation operation;

FIG. 5 illustrates compensation operation;

FIG. 6 illustrates compensation operation;

FIG. 7 illustrates compensation operation;

FIG. 8 illustrates compensation operation;

FIGS. 9A and 9B illustrate circuits of an imaging device;

FIGS. 10A and 10B are cross-sectional views of imaging devices includingcircuit portions;

FIGS. 11A to 11C are cross-sectional views of imaging devices includingcircuit portions;

FIGS. 12A to 12D are cross-sectional views each illustrating aconnection configuration of a photoelectric conversion element;

FIGS. 13A and 13B are cross-sectional views each illustrating aconnection configuration of a photoelectric conversion element;

FIG. 14 is a cross-sectional view of an imaging device including acircuit portion;

FIGS. 15A to 15F are cross-sectional views each illustrating aconnection configuration of a photoelectric conversion element;

FIGS. 16A1, 16A2, 16A3, 16B1, 16B2, and 16B3 illustrate a bent imagingdevice;

FIGS. 17A and 17B each illustrate a structure of an imaging device;

FIG. 18 is a timing chart illustrating operation of an imaging device;

FIG. 19 is a timing chart illustrating operation of an imaging device;

FIGS. 20A and 20B are timing charts illustrating operations of a globalshutter system and a rolling shutter system, respectively;

FIGS. 21A and 21B are a top view and a cross-sectional view of atransistor;

FIGS. 22A and 22B are a top view and a cross-sectional view of atransistor;

FIGS. 23A and 23B are a top view and a cross-sectional view of atransistor;

FIGS. 24A and 24B are a top view and a cross-sectional view of atransistor;

FIGS. 25A and 25B are a top view and a cross-sectional view of atransistor;

FIGS. 26A and 26B are a top view and a cross-sectional view of atransistor;

FIGS. 27A to 27D are cross-sectional views of transistors in the channelwidth direction;

FIGS. 28A to 28F are each a cross-sectional view of a transistor in thechannel length direction;

FIGS. 29A to 29C are a top view and cross-sectional views illustrating asemiconductor layer;

FIGS. 30A to 30C are a top view and cross-sectional views illustrating asemiconductor layer;

FIGS. 31A and 31B are a top view and a cross-sectional view illustratinga transistor;

FIGS. 32A and 32B are a top view and a cross-sectional view illustratinga transistor;

FIGS. 33A and 33B are a top view and a cross-sectional view illustratinga transistor;

FIGS. 34A and 34B are a top view and a cross sectional view illustratinga transistor;

FIGS. 35A and 35B are a top view and a cross-sectional view illustratinga transistor;

FIGS. 36A and 36B are a top view and a cross-sectional view illustratinga transistor;

FIGS. 37A to 37D are cross-sectional views of transistors in the channelwidth direction;

FIGS. 38A to 38F are each a cross-sectional view of a transistor in thechannel length direction;

FIGS. 39A and 39B are each a top view illustrating a transistor;

FIGS. 40A to 40C illustrate a method for manufacturing a transistor;

FIGS. 41A to 41C illustrate a method for manufacturing a transistor;

FIGS. 42A to 42C illustrate a method for manufacturing a transistor;

FIGS. 43A to 43C illustrate a method for manufacturing a transistor;

FIGS. 44A to 44C illustrate a method for manufacturing a transistor;

FIGS. 45A to 45C illustrate a method for manufacturing a transistor;

FIGS. 46A to 46F illustrate electronic devices; and

FIG. 47 illustrates a circuit of an imaging device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription. It will be readily appreciated by those skilled in the artthat modes and details of the present invention can be modified invarious ways without departing from the spirit and scope of the presentinvention. The present invention therefore should not be construed asbeing limited to the following description of the embodiments. Instructures of the invention described below, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and the description thereof is notrepeated in some cases. The same components are denoted by differenthatching patterns in different drawings, or the hatching patterns areomitted in some cases.

For example, in this specification and the like, an explicit description“X and Y are connected” means that X and Y are electrically connected, Xand Y are functionally connected, and X and Y are directly connected.Accordingly, without being limited to a predetermined connectionrelationship, for example, a connection relationship shown in drawingsor texts, another connection relationship is included in the drawings orthe texts.

Here, each of X and Y denotes an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer).

Examples of the case where X and Y are directly connected include thecase where an element that enables electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) is notconnected between X and Y, and the case where X and Y are connectedwithout the element that enables electrical connection between X and Yprovided therebetween.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. Note that the switch is controlled to beturned on or off That is, the switch is conducting or not conducting (isturned on or off) to determine whether current flows therethrough ornot. Alternatively, the switch has a function of selecting and changinga current path. Note that the case where X and Y are electricallyconnected includes the case where X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a D/A converter circuit, anA/D converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; astorage circuit; or a control circuit) can be connected between X and Y.Note that for example, in the case where a signal output from X istransmitted to Y even when another circuit is provided between X and Y,X and Y are functionally connected. The case where X and Y arefunctionally connected includes the case where X and Y are directlyconnected and X and Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the explicit description “X and Yare connected.”

For example, the case where a source (or a first terminal or the like)of a transistor is electrically connected to X through (or not through)Z1 and a drain (or a second terminal or the like) of the transistor iselectrically connected to Y through (or not through) Z2, or the casewhere a source (or a first terminal or the like) of a transistor isdirectly connected to part of Z1 and another part of Z1 is directlyconnected to X while a drain (or a second terminal or the like) of thetransistor is directly connected to part of Z2 and another part of Z2 isdirectly connected to Y, can be expressed by using any of the followingexpressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in that order,” “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin that order,” and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are connected in that order.” When theconnection order in a circuit structure is defined by an expressionsimilar to the above examples, a source (or a first terminal or thelike) and a drain (or a second terminal or the like) of a transistor canbe distinguished from each other to specify the technical scope.

Other examples of the expressions include “a source (or a first terminalor the like) of a transistor is electrically connected to X through atleast a first connection path, the first connection path does notinclude a second connection path, the second connection path is a pathbetween the source (or the first terminal or the like) of the transistorand a drain (or a second terminal or the like) of the transistor, Z1 ison the first connection path, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leasta third connection path, the third connection path does not include thesecond connection path, and Z2 is on the third connection path.” It isalso possible to use the expression “a source (or a first terminal orthe like) of a transistor is electrically connected to X through atleast Z1 on a first connection path, the first connection path does notinclude a second connection path, the second connection path includes aconnection path through the transistor, a drain (or a second terminal orthe like) of the transistor is electrically connected to Y through atleast Z2 on a third connection path, and the third connection path doesnot include the second connection path.” Still another example of theexpressions is “a source (or a first terminal or the like) of atransistor is electrically connected to X through at least Z1 on a firstelectrical path, the first electrical path does not include a secondelectrical path, the second electrical path is an electrical path fromthe source (or the first terminal or the like) of the transistor to adrain (or a second terminal or the like) of the transistor, the drain(or the second terminal or the like) of the transistor is electricallyconnected to Y through at least Z2 on a third electrical path, the thirdelectrical path does not include a fourth electrical path, and thefourth electrical path is an electrical path from the drain (or thesecond terminal or the like) of the transistor to the source (or thefirst terminal or the like) of the transistor.” When the connection pathin a circuit structure is defined by an expression similar to the aboveexamples, a source (or a first terminal or the like) and a drain (or asecond terminal or the like) of a transistor can be distinguished fromeach other to specify the technical scope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, the term “electrical connection” in thisspecification also means such a case where one conductive film hasfunctions of a plurality of components.

Note that the terms “film” and “layer” can be interchanged with eachother depending on circumstances or conditions. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. In addition, the term “insulating film” can be changed intothe term “insulating layer” in some cases.

Embodiment 1

In this embodiment, an imaging device that is one embodiment of thepresent invention is described with reference to drawings.

An imaging device in one embodiment of the present invention includes apixel circuit that can compensate variation in threshold voltage of asource follower amplifier transistor in a pixel of an imaging devicethat outputs signal charge (data) by a source follower.

FIG. 1 is a circuit diagram of a circuit 11 that can function as a pixelcircuit and a circuit 12 that can function as a reference current sourcecircuit included in an imaging device in one embodiment of the presentinvention. In FIG. 1 and the like, transistors are n-ch transistors;however, one embodiment of the present invention is not limited thereto.The transistors may be p-ch transistors by reversing the magnituderelationship of a potential as illustrated in FIG. 47. Alternatively,some of the n-ch transistors may be replaced with p-ch transistors.

The circuit 11 is broadly divided into a photoelectric conversionportion and a signal generation portion. The photoelectric conversionportion includes a photodiode 60, a transistor 51, and a transistor 52.The signal generation portion includes a transistor 53, a transistor 54,a transistor 55, a transistor 56, a transistor 57, a capacitor C1, acapacitor C2, a capacitor C3, and a capacitor C4. Note that thecapacitor C4 can be omitted.

The circuit 12 includes a transistor 58 and an output terminal (OUT).

The circuit 12 connected to a wiring 31 can have a structure illustratedin FIG. 2. In FIG. 2, a transistor 59 is added to the circuit 12. Thiscircuit is a current mirror circuit.

In the circuit 11 in FIG. 1, one terminal of the photodiode 60 iselectrically connected to one of a source and a drain of the transistor51. The other of the source and the drain of the transistor 51 iselectrically connected to one of a source and a drain of the transistor52. The other of the source and the drain of the transistor 51 iselectrically connected to one terminal of the capacitor C1. One of asource and a drain of the transistor 53 is electrically connected to theother terminal of the capacitor C1. The other terminal of the capacitorC1 is electrically connected to one terminal of the capacitor C2. One ofa source and a drain of the transistor 54 is electrically connected tothe other terminal of the capacitor C2. The other of the source and thedrain of the transistor 54 is electrically connected to one of a sourceand a drain of the transistor 55. One of a source and a drain of thetransistor 56 is electrically connected to one of the source and thedrain of the transistor 55. One terminal of the capacitor C3 iselectrically connected to the other terminal of the capacitor C2. Theother terminal of the capacitor C3 is electrically connected to theother of the source and the drain of the transistor 56. A gate of thetransistor 56 is electrically connected to one terminal of the capacitorC3. One of a source and a drain of the transistor 57 is electricallyconnected to the other of the source and the drain of the transistor 56.The other of the source and the drain of the transistor 57 iselectrically connected to one of a source and a drain of the transistor58.

The other terminal of the photodiode 60 is electrically connected to awiring 21 (VPD). The other of the source and the drain of the transistor52 is electrically connected to a wiring 22 (VPR). The other of thesource and the drain of the transistor 55 is electrically connected to awiring 23 (VPI). A gate of the transistor 51 is electrically connectedto a wiring 25 (TX). A gate of the transistor 52 is electricallyconnected to a wiring 26 (PR). A gate of the transistor 53 iselectrically connected to a wiring 27 (W). A gate of the transistor 54is electrically connected to a wiring 28 (AZ). A gate of the transistor55 is electrically connected to a wiring 29 (PSW). A gate of thetransistor 57 is electrically connected to a wiring 30 (SE). The otherof the source and the drain of the transistor 57 is electricallyconnected to the wiring 31.

In the circuit 12, one of the source and the drain of the transistor 58is electrically connected to the wiring 31, and the other of the sourceand the drain of the transistor 58 is electrically connected to a wiring24 (VPO). A gate of the transistor 58 is electrically connected to awiring 32 (BR). The output terminal (OUT) is provided in one of thesource and the drain of the transistor 58.

Here, the wiring 21 (VPD), the wiring 22 (VPR), the wiring 23 (VPI), andthe wiring 24 (VPO) can function as power supply lines. The wiring 25(TX), the wiring 26 (PR), the wiring 27 (W), the wiring 28 (AZ), thewiring 29 (PSW), the wiring 30 (SE), the wiring 31, and the wiring 32(BR) can function as signal lines.

In FIG. 1, the other of the source and the drain of the transistor 53 isconnected to the wiring 21 (VPD); however, the other of the source andthe drain of the transistor 53 may be connected to another wiring or thelike capable of supplying a fixed potential.

In addition, in FIG. 1, the other terminal of the capacitor C4 isconnected to the wiring 23 (VPI); however, the other terminal of thecapacitor C4 may be connected to another wiring or the like capable ofsupplying a fixed potential.

In the above structure, a node to which the other of the source and thedrain of the transistor 51, one of the source and the drain of thetransistor 52, and one terminal of the capacitor C1 are connected isdenoted by FD 1.

A node to which one of the source and the drain of the transistor 53,the other terminal of the capacitor C1, one terminal of the capacitorC2, and one terminal of the capacitor C4 are connected is denoted byFD2.

A node to which one of the source and the drain of the transistor 54,the other terminal of the capacitor C2, one terminal of the capacitorC3, and the gate of the transistor 56 are connected is denoted by AG.

A node to which the other of the source and the drain of the transistor56, the other terminal of the capacitor C3, and one of the source andthe drain of the transistor 57 are connected is denoted by AS.

A diode element formed using a silicon substrate with a pn junction or apin junction can be used as the photodiode 60. Alternatively, a pindiode element formed using an amorphous silicon film, a microcrystallinesilicon film, or the like may be used. Note that although the circuit 11includes the photodiode, the circuit 11 may include anotherphotoelectric conversion element. For example, a diode-connectedtransistor may be used. Alternatively, a variable resistor or the likeutilizing a photoelectric effect may be formed using silicon, germanium,selenium, or the like.

Alternatively, a photoelectric conversion element that includes seleniumutilizing a phenomenon called avalanche multiplication may be used. Inthe photoelectric conversion element, a highly sensitive sensor in whichthe amount of amplification of electrons with respect to the amount ofincident light is large can be obtained.

In the circuit 11, the photodiode 60 is a light-receiving element andcan have a function of generating current based on the amount of lightincident on the circuit 11. The transistor 51 can have a function ofcontrolling charge accumulation in the node FD1 performed by thephotodiode 60. The transistor 52 can have a function of executingoperation of resetting the potential of the node FD1. The transistor 53can have a function of executing operation of resetting the potential ofthe node FD2. The transistors 54 and 55 can each have a function ofsupplying current to the transistor 56. The transistor 56 can have afunction of executing operation of outputting a signal based on thepotential of the node AG. The transistor 57 can have a function ofexecuting operation of controlling selection of the circuit 11 (pixelcircuit) at the time of reading.

In the circuit 12, the transistor 58 can function as a current sourcetransistor. In addition, the transistor 58 can have a function ofoutputting a voltage signal based on current supplied to the transistor58 from the output terminal (OUT).

In the imaging device in one embodiment of the present invention withthe above structure, an output signal can be compensated when thecircuit 11 stores the threshold voltage of the transistor 56 included inthe circuit 11.

Details of the compensation operation and output operation aftercompensation are described with reference to a timing chart in FIG. 3.The timing chart in FIG. 3 shows the potentials of the wiring 24 (VPO),the wiring 25 (TX), the wiring 26 (PR), the wiring 27 (W), the wiring 28(AZ), the wiring 29 (PSW), the wiring 30 (SE), the wiring 32 (BR), thenode FD1, the node FD2, the node AG, and the node AS. Note that eachtransistor is turned on or off in accordance with a potential suppliedto a wiring connected to the gate of each transistor.

In the circuit diagram used for the description, transistors other thanthe transistor 56 are described as switches in order to clarifyconduction state of the transistors. In addition, some referencenumerals are omitted. Here, the wiring 21 (VPD) has a low potential(“GND”), the wiring 22 (VPR) has a high potential (“VPR”), and thewiring 23 (VPI) has a high potential (“VPI”).

At time T1, the transistors 52, 53, 54, 55, 57, and 58 are turned on andthe transistor 51 is turned off. The potential of the wiring 24 (VPO) isincreased to “V_(α),” and the potential of the wiring 32 (BR) isincreased to “BR+V_(β).” For example, “V_(α)” can be equal to “VPI/2”when GND is 0 V. At this time, comparatively high bias current flows tothe transistor 58, and the source potential of the transistor 56 (thepotential of the node AS) can be lower than the gate potential of thetransistor 56 (the potential of the node AG)—threshold voltage (V_(th))(see a current path indicated by a broken line in FIG. 4). In otherwords, the potential relationship in the circuit is the node AS<the nodeAG−V_(th).

Here, the potential of the node FD1 is set to “VPR,” i.e., the potentialof the wiring 22 (VPR). The potential of the node FD2 is set to “GND”(for example, 0 V), i.e., the potential of the wiring 21 (VPD). Thepotential of the node AG is set to “VPI,” i.e., the potential of thewiring 23 (VPI). The potential of the node AS is set to “VPO,” i.e., thepotential of the wiring 24 (VPO).

Next, at time T2, the transistor 55 is turned off to shut off a biascurrent path and to discharge the node AG (see FIG. 5). At this time,the potential of the node AG is changed from “VPI” to “VPO+V_(th)” anddischarging is terminated.

Then, at time T3, the transistor 54 is turned off to hold the thresholdvoltage (V_(th)) of the transistor 56 in the capacitor C3 (see FIG. 6).

At time T4, after the transistor 57 is turned off, compensation isterminated by turning on the transistor 55, turning off the transistor53, and setting the potential of the wiring 32 (BR) to “BR” (see FIG.7).

Next, on the assumption of actual imaging operation, operation when thepotential of the node FD2 is changed by V_(α), is described. First, inorder to change the potential of the node FD2 by V_(α), the transistor52 is turned off, the transistor 51 is turned on while the potential“VPR” of the wiring 22 (VPR) is held in the node FD1, and chargecorresponding to V_(α), is discharged to the wiring 21 (VPD) through thephotodiode 60 irradiated with light. Then, the transistor 51 is turnedoff to hold the potential of the node FD1. Through the above operation,the potential of the node FD1 can be changed from “VPR” to “VPR−V_(α).”

When the potential of the node FD1 is changed from “VPR” to “VPR−V_(α),”the potential of the node FD2 is changed from “VPD (=0 V)” to “−V_(α).”In addition, the potential of the node AG is changed from “VPO+V_(th)”to “VPO−V_(α)+V_(th).”

Here, current based on the potential of the wiring 32 (BR) can besupplied to the transistor 58, and bias current is supplied to thetransistor 56 when the transistor 57 is turned on. When a differencebetween a gate potential and a source potential for supplying the biascurrent to the transistor 56 at this time is denoted by “V_(gs)”(=V_(th)+V_(gs)), the potential of the node AS is(VPO−V_(α)+V_(th))−(V_(th)+V_(gs))=VPO−V_(α)−V_(gs) (see FIG. 8). Thefollowing drain current equation (13 is a constant) indicates that thedrain current (I_(d)) supplied to the transistor 56 does not depend onthe threshold voltage (V_(th)).

$\begin{matrix}\begin{matrix}{{Id} = {{\beta/2}\left( {{Vgs}^{\prime} - {Vth}} \right)^{2}}} \\{= {{\beta/2}\left( {{Vth} + {Vgs} - {Vth}} \right)^{2}}} \\{= {{\beta/2} \cdot {Vgs}^{2}}}\end{matrix} & (1)\end{matrix}$

Thus, a signal that does not include variation in the threshold voltageof the transistor 56 can be output to the output terminal (OUT), andhigh-quality imaging data can be obtained. Note that in the circuitstructure of FIG. 1, as the intensity of light delivered to thephotodiode 60 becomes higher, a signal output from the output terminal(OUT) becomes smaller.

It is not necessary to perform the compensation operation by eachimaging and imaging can be successively performed only by onecompensation operation. Needless to say, the compensation operation maybe performed before imaging, after imaging, at the time of power-on, atthe time of power-off, or at given timing using a timer or the like.

An imaging device in one embodiment of the present invention may have astructure in FIG. 9A or 9B. The connection direction of the photodiode60 of the photoelectric conversion portion in the circuit 11 in FIG. 9Ais opposite to that in FIG. 1. In that case, the wiring 21 (VPD) has ahigh potential and the wiring 22 (VPR) has a low potential. The circuitdescription in FIG. 1 can be referred to for compensation operation andoutput operation. In that case, as the intensity of light delivered tothe photodiode 60 becomes higher, the potential of the node FD1 becomeshigher. Thus, in the circuit structure of FIG. 9A, as the intensity oflight delivered to the photodiode 60 becomes higher, a signal outputfrom the output terminal (OUT) becomes larger.

In FIG. 9B, the transistor 52 is omitted from the circuit 11 in FIG. 1.In that case, the wiring 21 (VPD) can be changed to either a lowpotential or a high potential. FD1 reset operation can be performed whenthe wiring 21 (VPD) has a high potential. In a predetermined period,when the wiring 21 (VPD) has a high potential, forward bias is appliedto the photodiode 60. Thus, the potential of the node FD1 can be set tothe potential of the wiring 21 (VPD).

In the case where light detection operation (accumulation operation) isperformed, the potential of the wiring 21 (VPD) is set to a lowpotential. When the wiring 21 (VPD) has a low potential, reverse bias isapplied to the photodiode 60; thus, charge can be released from the nodeFD1 to the wiring 21 (VPD) in accordance with light intensity. In thatcase, as the intensity of light delivered to the photodiode 60 becomeshigher, the potential of the node FD1 becomes lower. Thus, in thecircuit structure of FIG. 9B, as the intensity of light delivered to thephotodiode 60 becomes higher, a signal output from the output terminal(OUT) becomes smaller.

In the imaging device in one embodiment of the present invention, atransistor including an oxide semiconductor is preferably used. The useof the transistor including an oxide semiconductor in the circuit 11 canbroaden the dynamic range of imaging. In the circuit structure in FIG.1, when the intensity of light entering the photodiode 60 is high, thepotential of the node AG becomes lower. Since the transistor includingan oxide semiconductor has extremely low off-state current, currentbased on the potential of the node AG (the gate potential of thetransistor 56) can be accurately output even when the gate potential isextremely low. Thus, it is possible to broaden the detectable range ofilluminance, i.e., the dynamic range.

A period during which charge can be held in the node FD1, the node FD2,the node AG, and the node AS can be extremely long owing to the lowoff-state current of the transistor including an oxide semiconductor.Thus, a global shutter system, in which charge accumulation operation isperformed in all the pixels substantially at the same time, can be usedwithout a complicated circuit structure and operation method. Therefore,an image with little distortion can be easily obtained even in the caseof a moving object. Furthermore, exposure time (a period of performingcharge accumulation operation) can be long; thus, the imaging device issuitable for imaging even in a low illuminance environment.

A transistor connected to any of the node FD1, the node FD2, the nodeAG, and the node AS needs to be a transistor with low noise. The channelof a transistor including two or three oxide semiconductor layers to bedescribed later is a buried channel, which has significantly highresistance to noise. Thus, the use of the transistor leads to an imagewith little noise.

In the one embodiment of the present invention, an output signal thatdoes not depend on variation in the threshold voltage (V_(th)) of theamplifier transistor (the transistor 56) included in the pixel circuitcan be obtained.

FIG. 10A is an example of a cross-sectional view of an imaging deviceincluding a circuit portion. A circuit portion 90 is a combination of atransistor 70 that includes an active region in a silicon substrate 40and a transistor 71 that includes an oxide semiconductor as an activelayer, and can form, for example, an inverter circuit or a memorycircuit. In addition, a circuit portion 92 is a combination of thephotodiode 60 formed using the silicon substrate 40 and the transistor51 that includes an oxide semiconductor as an active layer, andcorresponds to part of the photoelectric conversion portion of thecircuit 11 in FIG. 1. Note that wirings and contact plugs indicated bybroken lines show that placement is different from that of other wiringsand contact plugs in a depth direction.

Although wirings, electrodes, and conductors 89 are illustrated asindependent components in the drawings in this embodiment, in the casewhere such components are electrically connected to each other, they maybe provided as one component. Moreover, the structure in which the gateelectrodes, the source electrodes, or the drain electrodes of thetransistors are connected to wirings through the conductors 89 is onlyan example, and there is a case in which the gate electrodes, the sourceelectrodes, and the drain electrodes of the transistors function aswirings. Furthermore, there are a case in which one or more of thewirings illustrated in the drawings are not provided and a case in whichanother wiring or transistor is included in any of the layers.

Insulating layers each functioning as a protective film, an interlayerinsulating film, or a planarization film are provided over thecomponents. Insulating layers 41 to 43 and the like (the insulatinglayer 41 is not illustrated in FIGS. 10A and 10B), for example, can eachbe formed using an inorganic insulating film such as a silicon oxidefilm or a silicon oxynitride film, an organic insulating film such as anacrylic resin film or a polyimide resin film, or the like. Top surfacesof the insulating layers 41 to 43 and the like are preferably subjectedto planarization treatment as necessary by chemical mechanical polishing(CMP) or the like.

In FIG. 10A, the photodiode 60 and the transistor 51 can be formed tooverlap with each other; thus, the integration degree of pixels can beincreased. In other words, the resolution of the imaging device can beincreased. Furthermore, since the silicon substrate 40 is not providedwith a transistor formed in the occupation area of the circuit portion92, the area of the photodiode can be large. Thus, an image with lownoise can be obtained even in a low illuminance environment.

Although FIGS. 10A and 10B illustrate a structure in which thephotodiode 60 and the transistor 70 are formed using the siliconsubstrate 40, one embodiment of the present invention is not limitedthereto. For example, the transistor 70 may be formed using the siliconsubstrate 40 and a photodiode formed using another substrate may beattached. Alternatively, the transistor 70 may be formed without the useof the silicon substrate 40, and a transistor that includes an oxidesemiconductor as an active layer may be provided as in the transistors71 and 51. Alternatively, as illustrated in FIG. 10B, the transistors 70and 51 may be provided using the silicon substrate 40. An element otherthan the transistor 70 may be formed using the silicon substrate 40. Forexample, a capacitor, a diode, or a resistor may be formed using thesilicon substrate 40.

In the structure in FIG. 10A, an insulating layer 95 is provided betweena region including the transistor 70 and the photodiode 60 and a regionincluding the transistors 71 and 51.

Dangling bonds of silicon are terminated with hydrogen in insulatinglayers provided in the vicinity of the active region of the transistor70. Therefore, hydrogen has an effect of improving the reliability ofthe transistor 70. Meanwhile, hydrogen in insulating layers provided inthe vicinities of oxide semiconductor layers that are the active layersof the transistors 51 and 71 and the like causes generation of carriersin the oxide semiconductors. Therefore, hydrogen might reduce thereliability of the transistors 51 and 71 and the like. Consequently, inthe case where one layer that includes a transistor including asilicon-based semiconductor material and the other layer that includesthe transistor including an oxide semiconductor are stacked, it ispreferable that the insulating layer 95 having a function of preventingdiffusion of hydrogen be provided between these layers. Hydrogen isconfined in the one layer by the insulating layer 95, so that thereliability of the transistor 51 can be improved. Furthermore, diffusionof hydrogen from the one layer to the other layer is inhibited, so thatthe reliability of the transistors 51 and 71 and the like can beimproved.

The insulating layer 95 can be formed using aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, hafnium oxynitride, or yttria-stabilizedzirconia (YSZ), for example.

The silicon substrate 40 is not limited to a bulk silicon substrate andmay be an SOI substrate. Furthermore, the silicon substrate 40 can bereplaced with a substrate made of germanium, silicon germanium, siliconcarbide, gallium arsenide, aluminum gallium arsenide, indium phosphide,gallium nitride, or an organic semiconductor, or a substrate over whicha thin film of the material is formed.

The transistor 70 can be a transistor of various types without beinglimited to a planar type transistor. For example, the transistor 70 canbe a fin-type transistor or a tri-gate transistor.

The transistor 51 can include various types of semiconductors as well asan oxide semiconductor depending on conditions. For example, thetransistor 51 can include silicon, germanium, silicon germanium, siliconcarbide, gallium arsenide, aluminum gallium arsenide, indium phosphide,gallium nitride, or an organic semiconductor.

The imaging device of one embodiment of the present invention may have astructure in FIG. 11A.

A variety of elements can be used as a photoelectric conversion element80 which is a modification example of the photodiode 60 illustrated inFIGS. 10A and 10B. FIG. 11A illustrates the photoelectric conversionelement 80 including a selenium-based material for a photoelectricconversion layer 81. The photoelectric conversion element 80 including aselenium-based material has high external quantum efficiency withrespect to visible light. In the photoelectric conversion element, ahighly sensitive sensor in which the amount of amplification ofelectrons with respect to the amount of incident light by avalanchemultiplication is large can be obtained. Moreover, the selenium-basedmaterial has a high light absorption coefficient, which leads to anadvantage that the photoelectric conversion layer 81 is easily formed tobe thin.

Amorphous selenium or crystalline selenium can be used as aselenium-based material. Crystalline selenium can be obtained by, forexample, depositing amorphous selenium and then performing heattreatment. When the crystal grain size of crystalline selenium issmaller than a pixel pitch, variation in characteristics between pixelscan be reduced. Moreover, crystalline selenium has higher spectralsensitivity and light-absorption coefficient than those of amorphousselenium.

Although the photoelectric conversion layer 81 is a single layer, astructure can be employed in which gallium oxide, cerium oxide, or thelike is used on a light-receiving surface side of a selenium-basedmaterial for a layer for preventing hole injection and nickel oxide,antimony sulfide, or the like is used for a layer for preventingelectron injection on an electrode 86 side.

Furthermore, the photoelectric conversion layer 81 may be a layerincluding a compound of copper, indium, and selenium (CIS).Alternatively, a layer including a compound of copper, indium, gallium,and selenium (CIGS) may be used. With the CIS layer or the CIGS layer, aphotoelectric conversion element that can utilize avalanchemultiplication in a manner similar to that of a single layer of seleniumcan be formed.

In the photoelectric conversion element 80 including a selenium-basedmaterial, for example, the photoelectric conversion layer 81 can beprovided between a light-transmitting conductive layer 82 and theelectrode 86 formed using a metal material or the like. CIS and CIGS arep-type semiconductors and may be formed in contact with an n-typesemiconductor such as cadmium sulfide or zinc sulfide to form ajunction.

Comparatively high voltage (e.g., 10 V or higher) is preferably appliedto the photoelectric conversion element to cause the avalanchephenomenon. Since the OS transistor has higher drain breakdown voltagethan the Si transistor, comparatively high voltage can be easily appliedto the photoelectric conversion element. Therefore, by combination ofthe OS transistor with high drain breakdown voltage and thephotoelectric conversion element in which the selenium-based material isused for the photoelectric conversion layer, a highly sensitive andhighly reliable imaging device can be obtained.

Although the photoelectric conversion layer 81 and thelight-transmitting conductive layer 82 are not divided between pixelcircuits in FIG. 11A, the photoelectric conversion layer 81 and thelight-transmitting conductive layer 82 may be divided between circuitsas illustrated in FIG. 12A. Furthermore, a partition wall 77 formedusing an insulator is preferably provided in a region between pixelswhere the electrode 86 is not provided so as not to generate a crack inthe photoelectric conversion layer 81 and the light-transmittingconductive layer 82; however, the partition wall 77 is not necessarilyprovided as illustrated in FIG. 12B. Although the light-transmittingconductive layer 82 and a wiring 87 are connected through a wiring 88and the conductor 89 in FIG. 11A, the light-transmitting conductivelayer 82 and the wiring 87 may be in direct contact with each other asin FIGS. 12C and 12D.

The electrode 86, the wiring 87, and the like may each be a multilayer.For example, as illustrated in FIG. 13A, the electrode 86 can includetwo conductive layers 86 a and 86 b and the wiring 87 can include twoconductive layers 87 a and 87 b. In the structure of FIG. 13A, forexample, the conductive layers 86 a and 87 a may be made of alow-resistance metal or the like, and the conductive layers 86 b and 87b may be made of a metal or the like that exhibits an excellent contactproperty with the photoelectric conversion layer 81. Such a structureimproves the electrical properties of the photoelectric conversionelement. Furthermore, even when the conductive layer 87 a contains ametal that causes electrolytic corrosion, which occurs when some kindsof metal are in contact with the light-transmitting conductive layer 82,electrolytic corrosion can be prevented because the conductive layer 87b is placed between the conductive layer 87 a and the light-transmittingconductive layer 82.

The conductive layers 86 b and 87 b can be formed using, for example,molybdenum, tungsten, or the like. The conductive layers 86 a and 87 acan be formed using, for example, aluminum, titanium, or a stack oftitanium, aluminum, and titanium that are layered in that order.

The insulating layer 41 and the like may each be a multilayer. Forexample, as illustrated in FIG. 13B, the conductor 89 has a differencein level in the case where the insulating layer 41 includes insulatinglayers 41 a and 41 b that have different etching rates. In the casewhere another insulating layer used as an interlayer insulating film ora planarization film is a multilayer, the conductor 89 also has adifference in level. Although the insulating layer 41 is formed usingtwo layers here, the insulating layer 41 and another insulating layermay each be formed using three or more layers.

Note that the partition wall 77 can be formed using an inorganicinsulator, an insulating organic resin, or the like. The partition wall77 may be colored black or the like in order to shield the transistorsand the like from light and/or to determine the area of alight-receiving portion in each pixel.

Note that although each of the transistors 51 and 71 includes a backgate in FIG. 11A, as illustrated in FIG. 11B, each of the transistors 51and 71 does not necessarily include a back gate. Alternatively, asillustrated in FIG. 11C, one or more transistors, for example, only thetransistor 51 may include a back gate. The back gate might beelectrically connected to a front gate of an opposite transistor.Alternatively, different fixed potentials might be supplied to the backgate and the front gate. Note that the presence or absence of the backgate can also be applied to another imaging device described in thisembodiment.

Alternatively, a PIN diode element or the like formed using an amorphoussilicon film, a microcrystalline silicon film, or the like may be usedas the photoelectric conversion element 80.

FIG. 14 illustrates an example in which a thin film PIN photodiode isused as the photoelectric conversion element 80. In the photodiode, ann-type semiconductor layer 85, an i-type semiconductor layer 84, and ap-type semiconductor layer 83 are stacked in that order. The i-typesemiconductor layer 84 is preferably formed using amorphous silicon. Thep-type semiconductor layer 83 and the n-type semiconductor layer 85 caneach be formed using amorphous silicon, microcrystalline silicon, or thelike that includes a dopant imparting the corresponding conductivitytype. A photodiode in which a photoelectric conversion layer is formedusing amorphous silicon has high sensitivity in a visible lightwavelength region, and therefore can easily sense weak visible light.

In the photoelectric conversion element 80 in FIG. 14, the n-typesemiconductor layer 85 functioning as a cathode is electricallyconnected to the electrode 86 that is electrically connected to thetransistor 56. Furthermore, the p-type semiconductor layer 83functioning as an anode is electrically connected to the wirings 88 and87 through the conductor 89.

In any case, the photoelectric conversion element 80 is preferablyformed so that the p-type semiconductor layer 83 serves as alight-receiving surface. When the p-type semiconductor layer 83 servesas a light-receiving surface, the output current of the photoelectricconversion element 80 can be increased. Therefore, as for the connectionconfiguration of the photoelectric conversion element 80 with wirings orthe like, what the anode is connected to and what the cathode isconnected to in FIG. 14 may be reversed.

Furthermore, any of examples illustrated in FIGS. 15A to 15F may beapplied to the structure of the photoelectric conversion element 80having a configuration of a PIN thin film photodiode and the connectionbetween the photoelectric conversion element 80 and the wirings. Notethat the structure of the photoelectric conversion element 80 and theconnection between the photoelectric conversion element 80 and thewirings are not limited thereto, and other configurations may beapplied.

FIG. 15A illustrates a structure provided with the light-transmittingconductive layer 82 in contact with the p-type semiconductor layer 83 ofthe photoelectric conversion element 80. The light-transmittingconductive layer 82 functions as an electrode and can increase theoutput current of the photoelectric conversion element 80.

For the light-transmitting conductive layer 82, the following can beused: indium tin oxide; indium tin oxide containing silicon; indiumoxide containing zinc; zinc oxide; zinc oxide containing gallium; zincoxide containing aluminum; tin oxide; tin oxide containing fluorine; tinoxide containing antimony; graphene; or the like. The light-transmittingconductive layer 82 is not limited to a single layer, and may be astacked layer of different films.

FIG. 15B illustrates a structure in which the p-type semiconductor layer83 of the photoelectric conversion element 80 is directly connected tothe wiring 88.

FIG. 15C illustrates a structure in which the light-transmittingconductive layer 82 in contact with the p-type semiconductor layer 83 ofthe photoelectric conversion element 80 is provided, and the wiring 88is electrically connected to the light-transmitting conductive layer 82.

FIG. 15D illustrates a structure in which an opening portion exposingthe p-type semiconductor layer 83 is provided in an insulating layercovering the photoelectric conversion element 80, and thelight-transmitting conductive layer 82 that covers the opening portionis electrically connected to the wiring 88.

FIG. 15E illustrates a structure provided with the conductor 89 thatpenetrates the photoelectric conversion element 80. In the structure,the wiring 87 is electrically connected to the p-type semiconductorlayer 83 through the conductor 89. Note that in the drawing, the wiring87 appears to be electrically connected to the electrode 86 through then-type semiconductor layer 85. However, resistance in the lateraldirection of the n-type semiconductor layer 85 is high; therefore, whenan appropriate distance is provided between the wiring 87 and theelectrode 86, the resistance between the wiring 87 and the electrode 86is extremely high. Thus, the photoelectric conversion element 80 canhave diode characteristics without a short circuit between the anode andthe cathode. Note that two or more conductors 89 that are electricallyconnected to the p-type semiconductor layer 83 may be provided.

FIG. 15F illustrates a structure in which the photoelectric conversionelement 80 in FIG. 15E is provided with the light-transmittingconductive layer 82 in contact with the p-type semiconductor layer 83.

Note that each of the photoelectric conversion elements 80 in FIGS. 15Dto 15F has an advantage of having a large light-receiving area becausewirings and the like do not overlap with a light-receiving region.

The photoelectric conversion element 80 formed using the selenium-basedmaterial, amorphous silicon, or the like can be formed through generalsemiconductor manufacturing processes such as a deposition process, alithography process, and an etching process. Furthermore, the resistanceof the selenium-based material is high; thus, a structure in which thephotoelectric conversion layer 81 is not divided between the circuitscan be employed as illustrated in FIG. 11A. Therefore, the imagingdevice in one embodiment of the present invention can be manufacturedwith a high yield at low cost. Meanwhile, to form a photodiode in whichthe photoelectric conversion layer 81 is formed using crystallinesilicon, processes with high difficulty, such as a polishing process anda bonding process, are needed.

As illustrated in FIGS. 16A1 and 16B1, the imaging device may be bent.FIG. 16A1 illustrates a state in which the imaging device is bent in thedirection of dashed-two dotted line X1-X2. FIG. 16A2 is across-sectional view illustrating a portion indicated by dashed-twodotted line X1-X2 in FIG. 16A1. FIG. 16A3 is a cross-sectional viewillustrating a portion indicated by dashed-two dotted line Y1-Y2 in FIG.16A1.

FIG. 16B1 illustrates a state where the imaging device is bent in thedirection of dashed-two dotted line X3-X4 and the direction ofdashed-two dotted line Y3-Y4. FIG. 16B2 is a cross-sectional viewillustrating a portion indicated by dashed-two dotted line X3-X4 in FIG.16B1. FIG. 16B3 is a cross-sectional view illustrating a portionindicated by dashed-two dotted line Y3-Y4 in FIG. 16B1.

Bending the imaging device can reduce field curvature and astigmatism.Thus, the optical design of lens and the like, which are used incombination of the imaging device, can be facilitated. For example, thenumber of lenses used for aberration correction can be reduced;accordingly, the size or weight of semiconductor devices including theimaging device can be easily reduced. In addition, the quality of acaptured image can be improved.

In this embodiment, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inthe other embodiments. Note that one embodiment of the present inventionis not limited thereto. Although an example in which one embodiment ofthe present invention is applied to an imaging device is described, oneembodiment of the present invention is not limited thereto. Depending oncircumstances or conditions, one embodiment of the present invention isnot necessarily applied to an imaging device. One embodiment of thepresent invention may be applied to a semiconductor device with ananother function, for example. Although examples in which, in oneembodiment of the present invention, a function of compensatingvariation or degradation in electrical characteristics of a transistoris provided or compensation operation is performed, one embodiment ofthe present invention is not limited thereto. Depending on circumstancesor conditions, one embodiment of the present invention does notnecessarily compensate variation or degradation in electricalcharacteristics of a transistor.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 2

In this embodiment, an example of a method for driving a pixel circuitis described.

The pixel circuit described in Embodiment 1 can perform first operationin which normal imaging is performed and second operation in whichdifferential data of imaging data of an initial frame and imaging dataof a current frame can be retained and a signal based on thedifferential data can be output. In the second operation, differentialdata can be output without a comparison process or the like in anexternal circuit; thus, the pixel circuit can be applied to a low-powersecurity camera or the like.

As illustrated in FIGS. 17A and 17B, an imaging device in one embodimentof the present invention includes a pixel portion 400 that includes thecircuits 11 arranged in a matrix, a row driver 410 connected to thecircuits 11, the circuits 12 connected to the circuits 11, an A/Dconverter 420 connected to the circuits 12, and a column driver 430connected to the A/D converter 420. The pixel portion 400 can have astructure in which the circuits 11 are arranged in a line as illustratedin FIG. 17B. In the imaging device having the structure illustrated inFIG. 17B, data in a plane can be read by scanning in one direction.

Imaging data obtained in the circuit 11 selected by the row driver 410is input to the A/D converter 420 through the circuit 12. The A/Dconverter 420 converts input imaging data into digital data by A/Dconversion. The A/D converted digital data are sequentially extracted tothe outside by the column driver 430. As the row driver 410 and thecolumn driver 430, for example, a variety of circuits such as a decoderand a shift register can be used.

Next, first operation of the circuit in FIG. 1 is described withreference to a timing chart in FIG. 18.

From the time T1 to the time T2, the wiring 25 (TX) is set to apotential higher than VPR+V_(th), the wiring 26 (PR) is set to apotential higher than VPR+V_(th), and the wiring 27 (W) is set to apotential higher than V_(th). At this time, the potential of the nodeFD1 is set to the potential of the wiring 22 (VPR) and the potential ofthe node FD2 is set to the potential of the wiring 21 (VPD), i.e., “GND”(reset operation).

From the time T2 to the time T3, the wiring 25 (TX) is set to apotential higher than VPR+V_(th), the wiring 26 (PR) is set to “GND,”and the wiring 27 (W) is set to a potential lower than −VPR. Here, thepotentials of the node FD1 and the node FD2 are decreased in response tolight with which the photodiode 60 is irradiated. When the amount ofdecrease in the potential of the node FD1 at the time T3 is denoted byV1, the potential of the node FD1 is VPR−V1. In addition, the potentialof the node FD2 is decreased by V2 due to capacitive coupling andbecomes GND−V2 (accumulation operation). Note that in the circuitstructure in FIG. 1, as the intensity of light delivered to thephotodiode 60 becomes higher, the potentials of the node FD1 and thenode FD2 become lower.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR)is set to “GND,” and the wiring 27 (W) is set to a potential lower than−VPR from the time T3 to the time T4, the potentials of the node FD1 andthe node FD2 are held.

In the case where the wiring 30 (SE) is set to a potential higher thanVPI+V_(th), from the time T4 to time T5, a signal based on imaging datais output to the output terminal (OUT) in accordance with the potentialof the node FD2 (selection operation). Through the operation from thetime T1 to the time T5, the first operation can be performed.

Next, second operation of the circuit in FIG. 1 is described withreference to a timing chart in FIG. 19.

From the time T1 to the time T2, the wiring 25 (TX) is set to apotential higher than VPR+V_(th), the wiring 26 (PR) is set to apotential higher than VPR+V_(th), and the wiring 27 (W) is set to apotential higher than V_(th). At this time, the potential of the nodeFD1 is set to the potential of the wiring 22 (VPR) and the potential ofthe node FD2 is set to the potential of the wiring 21 (VPD), i.e.,“GND.”

From the time T2 to the time T3, the wiring 25 (TX) is set to apotential higher than VPR+V_(th), the wiring 26 (PR) is set to “GND,”and the wiring 27 (W) is set to a potential higher than V_(th). Here,the potential of the node FD1 is decreased in response to light withwhich the photodiode 60 is irradiated. When the amount of decrease inthe potential of the node FD1 at the time T3 is denoted by V1, thepotential of the node FD1 is VPR−V1. Note that in the circuit structurein FIG. 1, as the intensity of light delivered to the photodiode 60becomes higher, the potential of the node FD1 becomes lower.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR)is set to “GND,” and the wiring 27 (W) is set to a potential higher thanV_(th) from the time T3 to the time T4, the potential of the node FD1 isheld.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR)is set to “GND,” and the wiring 27 (W) is set to a potential lower than−VPR from the time T4 to the time T5, the potentials of the node FD1 andthe node FD2 are held.

In the case where the wiring 25 (TX) is set to a potential higher thanVPR+V_(th), the wiring 26 (PR) is set to a potential higher thanVPR+V_(th), and the wiring 27 (W) is set to a potential lower than −VPRfrom the time T5 to time T6, the potential of the node FD1 is increasedby V1 and the potential of the node FD2 is increased by V2 due tocapacitive coupling. Here, V1 and V2 are potentials that reflectilluminance of an initial frame.

In the case where the wiring 25 (TX) is set to a potential higher thanVPR+V_(th), the wiring 26 (PR) is set to “GND,” and the wiring 27 (W) isset to a potential lower than −VPR from the time T6 to time T7, thepotentials of the node FD1 and the node FD2 are decreased in response tolight with which the photodiode 60 is irradiated. When the amount ofdecrease in the potential of the node FD1 at the time T6 is denoted byV1′, the potential of the node FD1 is VPR−V1′. In addition, thepotential of the node FD2 is decreased by V2′ due to capacitive couplingand becomes VPD+V2−V2′.

In the case where the wiring 25 (TX) is set to “GND,” the wiring 26 (PR)is set to “GND,” and the wiring 27 (W) is set to a potential lower than−VPR from the time T7 to time T8, the potentials of the node FD1 and thenode FD2 are held.

In the case where the wiring 30 (SE) is set to a potential higher thanVPI+V_(th) from the time T8 to time T9, a signal based on imaging datais output to the output terminal (OUT) in accordance with the potentialof the node FD2. In the above case, the potential of the node FD2 at thetime of signal output is GND+V2−V2′; thus, the potential is V2−V2′ whenGND is 0 V, for example. Here, V2 is a potential that reflects theilluminance of the initial frame, and V2′ is a potential that reflectsilluminance of a later frame (current frame). In other words, the secondoperation in which a difference between the initial frame and thecurrent frame is output can be performed.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 3

In this embodiment, an example of a method for driving a pixel circuitis described.

As described in Embodiment 2, the operation of the pixel circuit isrepetition of the reset operation, the accumulation operation, and theselection operation. As imaging modes in which the whole pixel matrix iscontrolled, a global shutter system and a rolling shutter system areknown.

FIG. 20A is a timing chart of a global shutter system. The timing chartillustrates operation of an imaging device in which a plurality of pixelcircuits in FIG. 1 are arranged in a matrix. Specifically, the timingchart illustrates operation of the pixel circuits from a first row to ann-th row (n is a natural number of 3 or more). The operation isdescribed giving the first operation described in Embodiment 2 as anexample.

In FIG. 20A, a signal 501, a signal 502, and a signal 503 are input tothe wirings 26 (PR) connected to the pixel circuits in the first row,the second row, and the n-th row, respectively. A signal 504, a signal506, and a signal 508 are input to the wirings 25 (TX) connected to thepixel circuits in the first row, the second row, and the n-th row,respectively. A signal 505, a signal 507, and a signal 509 are input tothe wirings 29 (PSW) connected to the pixel circuits in the first row,the second row, and the n-th row, respectively.

A period 510 is a period required for one imaging. A period 511 and aperiod 520 are periods in which reset operation and accumulationoperation are performed at the same time in the pixel circuits in eachrow, respectively. Note that the selection operation is sequentiallyperformed in the pixel circuits in each row. For example, in a period531, the selection operation is performed in the pixel circuits in thefirst row. As described above, in the global shutter system, the resetoperation and the accumulation operation are performed in all the pixelcircuits substantially at the same time, and then read operation issequentially performed in each row.

That is, in the global shutter system, since the accumulation operationis performed in all the pixel circuits substantially at the same time,imaging is simultaneously performed in the pixel circuits in all therows. Therefore, an image with little distortion can be obtained even inthe case of a moving object.

FIG. 20B is a timing chart of the case using a rolling shutter system.The description of FIG. 20A can be referred to for the signals 501 to509. A period 610 is a period required for one imaging. A period 611 isa period in which the pixels in the first row perform reset operation. Aperiod 612 is a period in which the pixels in the second row performreset operation. A period 613 is a period in which the pixels in then-th row perform reset operation. A period 621 is a period in which thepixels in the first row perform accumulation operation. A period 622 isa period in which the pixels in the second row perform accumulationoperation. A period 623 is a period in which the pixels in the n-th rowperform accumulation operation. A period 631 is a period in which thepixels in the first row perform selection operation. As described above,in the rolling shutter system, the accumulation operation is notperformed at the same time in all the pixels but is sequentiallyperformed in all the rows; thus, imaging is not simultaneously performedin the pixels in all the rows. Therefore, the timing of imaging in thefirst row is different from that of imaging in the last row, and thus animage with large distortion is obtained in the case of a moving object.

To achieve the global shutter system, the potential of a chargeaccumulation portion (the node FD2) needs to be held for a long timeuntil sequential reading of signals from the pixels is terminated. Whena transistor including a channel formation region formed using an oxidesemiconductor and having extremely low off-state current is used as thetransistor 55 or the like, the potential of the charge accumulationportion (the node FD2) can be held for a long time. In the case where atransistor including a channel formation region formed using silicon orthe like is used as the transistor 55 or the like, the potential of thecharge accumulation portion (the node FD2) cannot be held for a longtime because of high off-state current, which makes it difficult to usethe global shutter system.

As described above, the use of the transistor in which a channelformation region is formed using an oxide semiconductor for the pixelcircuits makes it easy to achieve the global shutter system. Note thatone embodiment of the present invention is not limited thereto.Depending on circumstances or conditions, a system other than the globalshutter system may be used. For example, depending on circumstances orconditions, the rolling shutter system may be used.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 4

In this embodiment, a transistor including an oxide semiconductor thatcan be used in one embodiment of the present invention is described withreference to drawings. In the drawings in this embodiment, somecomponents are enlarged, reduced in size, or omitted for easyunderstanding.

FIGS. 21A and 21B are a top view and a cross-sectional view illustratinga transistor 101 in one embodiment of the present invention. A crosssection in the direction of dashed-dotted line B1-B2 in FIG. 21A isillustrated in FIG. 21B. A cross section in the direction ofdashed-dotted line B3-B4 in FIG. 21A is illustrated in FIG. 27A. In somecases, the direction of dashed-dotted line B1-B2 is referred to as achannel length direction, and the direction of dashed-dotted line B3-B4is referred to as a channel width direction.

The transistor 101 includes an insulating layer 120 in contact with asubstrate 115; an oxide semiconductor layer 130 in contact with theinsulating layer 120; conductive layers 140 and 150 electricallyconnected to the oxide semiconductor layer 130; an insulating layer 160in contact with the oxide semiconductor layer 130 and the conductivelayers 140 and 150; a conductive layer 170 in contact with theinsulating layer 160; an insulating layer 175 in contact with theconductive layers 140 and 150, the insulating layer 160, and theconductive layer 170; and an insulating layer 180 in contact with theinsulating layer 175. The insulating layer 180 may function as aplanarization film as necessary.

Here, the conductive layer 140, the conductive layer 150, the insulatinglayer 160, and the conductive layer 170 can function as a sourceelectrode layer, a drain electrode layer, a gate insulating film, and agate electrode layer, respectively.

A region 231, a region 232, and a region 233 in FIG. 21B can function asa source region, a drain region, and a channel formation region,respectively. The region 231 and the region 232 are in contact with theconductive layer 140 and the conductive layer 150, respectively. When aconductive material that is easily bonded to oxygen is used for theconductive layers 140 and 150, for example, the resistance of theregions 231 and 232 can be reduced.

Specifically, since the oxide semiconductor layer 130 is in contact withthe conductive layers 140 and 150, an oxygen vacancy is generated in theoxide semiconductor layer 130, and interaction between the oxygenvacancy and hydrogen that remains in the oxide semiconductor layer 130or diffuses into the oxide semiconductor layer 130 from the outsidechanges the regions 231 and 232 to n-type regions with low resistance.

Note that functions of a “source” and a “drain” of a transistor aresometimes interchanged with each other when a transistor of an oppositeconductivity type is used or when the direction of current flow ischanged in circuit operation, for example. Therefore, the terms “source”and “drain” can be interchanged with each other in this specification.In addition, the term “electrode layer” can be changed into the term“wiring.”

The conductive layer 170 includes two layers, conductive layers 171 and172, but also may be a single layer or a stack of three or more layers.The same applies to other transistors described in this embodiment.

Each of the conductive layers 140 and 150 is a single layer, but alsomay be a stack of two or more layers. The same applies to othertransistors described in this embodiment.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 22A and 22B. FIG. 22A is a top view of atransistor 102. A cross section in the direction of dashed-dotted lineC1-C2 in FIG. 22A is illustrated in FIG. 22B. A cross section in thedirection of dashed-dotted line C3-C4 in FIG. 22A is illustrated in FIG.27B. In some cases, the direction of dashed-dotted line C1-C2 isreferred to as a channel length direction, and the direction ofdashed-dotted line C3-C4 is referred to as a channel width direction.

The transistor 102 has the same structure as the transistor 101 exceptthat an end portion of the insulating layer 160 functioning as a gateinsulating film is not aligned with an end portion of the conductivelayer 170 functioning as a gate electrode layer. In the transistor 102,wide areas of the conductive layers 140 and 150 are covered with theinsulating layer 160 and accordingly the resistance between theconductive layer 170 and the conductive layers 140 and 150 is high;therefore, the transistor 102 has low gate leakage current.

The transistors 101 and 102 each have a top-gate structure including aregion where the conductive layer 170 overlaps with the conductivelayers 140 and 150. To reduce parasitic capacitance, the width of theregion in the channel length direction is preferably greater than orequal to 3 nm and less than 300 nm. Since an offset region is not formedin the oxide semiconductor layer 130 in this structure, a transistorwith high on-state current can be easily formed.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 23A and 23B. FIG. 23A is a top view of atransistor 103. A cross section in the direction of dashed-dotted lineD1-D2 in FIG. 23A is illustrated in FIG. 23B. A cross section in thedirection of dashed-dotted line D3-D4 in FIG. 23A is illustrated in FIG.27A. In some cases, the direction of dashed-dotted line D1-D2 isreferred to as a channel length direction, and the direction ofdashed-dotted line D3-D4 is referred to as a channel width direction.

The transistor 103 includes the insulating layer 120 in contact with thesubstrate 115; the oxide semiconductor layer 130 in contact with theinsulating layer 120; the insulating layer 160 in contact with the oxidesemiconductor layer 130; the conductive layer 170 in contact with theinsulating layer 160; the insulating layer 175 covering the oxidesemiconductor layer 130, the insulating layer 160, and the conductivelayer 170; the insulating layer 180 in contact with the insulating layer175; and the conductive layers 140 and 150 electrically connected to theoxide semiconductor layer 130 through openings provided in theinsulating layers 175 and 180. The transistor 103 may further include,for example, an insulating layer (planarization film) in contact withthe insulating layer 180 and the conductive layers 140 and 150 asnecessary.

Here, the conductive layer 140, the conductive layer 150, the insulatinglayer 160, and the conductive layer 170 can function as a sourceelectrode layer, a drain electrode layer, a gate insulating film, and agate electrode layer, respectively.

The region 231, the region 232, and the region 233 in FIG. 23B canfunction as a source region, a drain region, and a channel formationregion, respectively. The regions 231 and 232 are in contact with theinsulating layer 175. When an insulating material containing hydrogen isused for the insulating layer 175, for example, the resistance of theregions 231 and 232 can be reduced.

Specifically, interaction between an oxygen vacancy generated in theregions 231 and 232 by the steps up to formation of the insulating layer175 and hydrogen that diffuses into the regions 231 and 232 from theinsulating layer 175 changes the regions 231 and 232 to n-type regionswith low resistance. As the insulating material containing hydrogen, forexample, silicon nitride, aluminum nitride, or the like can be used.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 24A and 24B. FIG. 24A is a top view of atransistor 104. A cross section in the direction of dashed-dotted lineE1-E2 in FIG. 24A is illustrated in FIG. 24B. A cross section in thedirection of dashed-dotted line E3-E4 in FIG. 24A is illustrated in FIG.27A. In some cases, the direction of dashed-dotted line E1-E2 isreferred to as a channel length direction, and the direction ofdashed-dotted line E3-E4 is referred to as a channel width direction.

The transistor 104 has the same structure as the transistor 103 exceptthat the conductive layers 140 and 150 in contact with the oxidesemiconductor layer 130 cover end portions of the oxide semiconductorlayer 130.

In FIG. 24B, regions 331 and 334 can function as a source region,regions 332 and 335 can function as a drain region, and a region 333 canfunction as a channel formation region.

The resistance of the regions 331 and 332 can be reduced in a mannersimilar to that of the regions 231 and 232 in the transistor 101.

The resistance of the regions 334 and 335 can be reduced in a mannersimilar to that of the regions 231 and 232 in the transistor 103. In thecase where the length of the regions 334 and 335 in the channel lengthdirection is less than or equal to 100 nm, preferably less than or equalto 50 nm, a gate electric field prevents a significant decrease inon-state current. Therefore, a reduction in resistance of the regions334 and 335 is not performed in some cases.

The transistors 103 and 104 each have a self-aligned structure that doesnot include a region where the conductive layer 170 overlaps with theconductive layers 140 and 150. A transistor with a self-alignedstructure, which has extremely low parasitic capacitance between a gateelectrode layer and source and drain electrode layers, is suitable forapplications that require high-speed operation.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 25A and 25B. FIG. 25A is a top view of atransistor 105. A cross section in the direction of dashed-dotted lineF1-F2 in FIG. 25A is illustrated in FIG. 25B. A cross section in thedirection of dashed-dotted line F3-F4 in FIG. 25A is illustrated in FIG.27A. In some cases, the direction of dashed-dotted line F1-F2 isreferred to as a channel length direction, and the direction ofdashed-dotted line F3-F4 is referred to as a channel width direction.

The transistor 105 includes the insulating layer 120 in contact with thesubstrate 115; the oxide semiconductor layer 130 in contact with theinsulating layer 120; conductive layers 141 and 151 electricallyconnected to the oxide semiconductor layer 130; the insulating layer 160in contact with the oxide semiconductor layer 130 and the conductivelayers 141 and 151; the conductive layer 170 in contact with theinsulating layer 160; the insulating layer 175 in contact with the oxidesemiconductor layer 130, the conductive layers 141 and 151, theinsulating layer 160, and the conductive layer 170; the insulating layer180 in contact with the insulating layer 175; and conductive layers 142and 152 electrically connected to the conductive layers 141 and 151,respectively, through openings provided in the insulating layers 175 and180. The transistor 105 may further include, for example, an insulatinglayer in contact with the insulating layer 180 and the conductive layers142 and 152 as necessary.

Here, the conductive layers 141 and 151 are in contact with the topsurface of the oxide semiconductor layer 130 and are not in contact withside surfaces of the oxide semiconductor layer 130.

The transistor 105 has the same structure as the transistor 101 exceptthat the conductive layers 141 and 151 are provided, that openings areprovided in the insulating layers 175 and 180, and that the conductivelayers 142 and 152 electrically connected to the conductive layers 141and 151, respectively, through the openings are provided. The conductivelayer 140 (the conductive layers 141 and 142) can function as a sourceelectrode layer, and the conductive layer 150 (the conductive layers 151and 152) can function as a drain electrode layer.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 26A and 26B. FIG. 26A is a top view of atransistor 106. A cross section in the direction of dashed-dotted lineG1-G2 in FIG. 26A is illustrated in FIG. 26B. A cross section in thedirection of dashed-dotted line G3-G4 in FIG. 26A is illustrated in FIG.27A. In some cases, the direction of dashed-dotted line G1-G2 isreferred to as a channel length direction, and the direction ofdashed-dotted line G3-G4 is referred to as a channel width direction.

The transistor 106 includes the insulating layer 120 in contact with thesubstrate 115; the oxide semiconductor layer 130 in contact with theinsulating layer 120; the conductive layers 141 and 151 electricallyconnected to the oxide semiconductor layer 130; the insulating layer 160in contact with the oxide semiconductor layer 130; the conductive layer170 in contact with the insulating layer 160; the insulating layer 175in contact with the insulating layer 120, the oxide semiconductor layer130, the conductive layers 141 and 151, the insulating layer 160, andthe conductive layer 170; the insulating layer 180 in contact with theinsulating layer 175; and the conductive layers 142 and 152 electricallyconnected to the conductive layers 141 and 151, respectively, throughopenings provided in the insulating layers 175 and 180. The transistor106 may further include, for example, an insulating layer (planarizationfilm) in contact with the insulating layer 180 and the conductive layers142 and 152 as necessary.

Here, the conductive layers 141 and 151 are in contact with the topsurface of the oxide semiconductor layer 130 and are not in contact withside surfaces of the oxide semiconductor layer 130.

The transistor 106 has the same structure as the transistor 103 exceptthat the conductive layers 141 and 151 are provided. The conductivelayer 140 (the conductive layers 141 and 142) can function as a sourceelectrode layer, and the conductive layer 150 (the conductive layers 151and 152) can function as a drain electrode layer.

In the structures of the transistors 105 and 106, the conductive layers140 and 150 are not in contact with the insulating layer 120. Thesestructures make the insulating layer 120 less likely to be deprived ofoxygen by the conductive layers 140 and 150 and facilitate oxygen supplyfrom the insulating layer 120 to the oxide semiconductor layer 130.

An impurity for forming an oxygen vacancy to increase conductivity maybe added to the regions 231 and 232 in the transistor 103 and theregions 334 and 335 in the transistors 104 and 106. As an impurity forforming an oxygen vacancy in an oxide semiconductor layer, for example,one or more of the following can be used: phosphorus, arsenic, antimony,boron, aluminum, silicon, nitrogen, helium, neon, argon, krypton, xenon,indium, fluorine, chlorine, titanium, zinc, and carbon. As a method foradding the impurity, plasma treatment, ion implantation, ion doping,plasma immersion ion implantation, or the like can be used.

When the above element is added as an impurity element to the oxidesemiconductor layer, a bond between a metal element and oxygen in theoxide semiconductor layer is cut, so that an oxygen vacancy is formed.Interaction between an oxygen vacancy in the oxide semiconductor layerand hydrogen that remains in the oxide semiconductor layer or is addedto the oxide semiconductor layer later can increase the conductivity ofthe oxide semiconductor layer.

When hydrogen is added to an oxide semiconductor in which an oxygenvacancy is formed by addition of an impurity element, hydrogen enters anoxygen vacant site and forms a donor level in the vicinity of theconduction band. Consequently, an oxide conductor can be formed. Here,an oxide conductor refers to an oxide semiconductor having become aconductor. Note that the oxide conductor has a light-transmittingproperty in a manner similar to the oxide semiconductor.

The oxide conductor is a degenerated semiconductor and it is suggestedthat the conduction band edge equals or substantially equals the Fermilevel. For that reason, an ohmic contact is made between an oxideconductor layer and conductive layers functioning as a source electrodelayer and a drain electrode layer; thus, contact resistance between theoxide conductor layer and the conductive layers functioning as a sourceelectrode layer and a drain electrode layer can be reduced.

The transistor in one embodiment of the present invention may include aconductive layer 173 between the oxide semiconductor layer 130 and thesubstrate 115 as illustrated in cross-sectional views in the channellength direction in FIGS. 28A to 28F and cross-sectional views in thechannel width direction in FIGS. 27C and 27D. When the conductive layer173 is used as a second gate electrode layer (back gate), the on-statecurrent can be increased or the threshold voltage can be controlled. Inthe cross-sectional views in FIGS. 28A to 28F, the width of theconductive layer 173 may be shorter than that of the oxide semiconductorlayer 130. Moreover, the width of the conductive layer 173 may beshorter than that of the conductive layer 170.

In order to increase the on-state current, for example, the conductivelayers 170 and 173 are made to have the same potential, and thetransistor is driven as a double-gate transistor. Furthermore, in orderto control the threshold voltage, a fixed potential that is differentfrom the potential of the conductive layer 170 is applied to theconductive layer 173. To set the conductive layers 170 and 173 at thesame potential, for example, as illustrated in FIG. 27D, the conductivelayers 170 and 173 may be electrically connected to each other through acontact hole.

Although the transistors 101 to 106 in FIGS. 21A and 21B, FIGS. 22A and22B, FIGS. 23A and 23B, FIGS. 24A and 24B, FIGS. 25A and 25B, and FIGS.26A and 26B are examples in which the oxide semiconductor layer 130 is asingle layer, the oxide semiconductor layer 130 may be a stacked layer.The oxide semiconductor layer 130 in the transistors 101 to 106 can bereplaced with the oxide semiconductor layer 130 in FIGS. 29A to 29C orFIGS. 30A to 30C.

FIGS. 29A to 29C are a top view and cross-sectional views of the oxidesemiconductor layer 130 with a two-layer structure. FIG. 29B illustratesa cross section in the direction of dashed-dotted line A1-A2 in FIG.29A. FIG. 29C illustrates a cross section in the direction ofdashed-dotted line A3-A4 in FIG. 29A.

FIGS. 30A to 30C are a top view and cross-sectional views of the oxidesemiconductor layer 130 with a three-layer structure. FIG. 30Billustrates a cross section in the direction of dashed-dotted line A1-A2in FIG. 30A. FIG. 30C illustrates a cross section in the direction ofdashed-dotted line A3-A4 in FIG. 30A.

Oxide semiconductor layers with different compositions, for example, canbe used as an oxide semiconductor layer 130 a, an oxide semiconductorlayer 130 b, and an oxide semiconductor layer 130 c.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 31A and 31B. FIG. 31A is a top view of atransistor 107. A cross section in the direction of dashed-dotted lineH1-H2 in FIG. 31A is illustrated in FIG. 31B. A cross section in thedirection of dashed-dotted line H3-H4 in FIG. 31A is illustrated in FIG.37A. In some cases, the direction of dashed-dotted line H1-H2 isreferred to as a channel length direction, and the direction ofdashed-dotted line H3-H4 is referred to as a channel width direction.

The transistor 107 includes the insulating layer 120 in contact with thesubstrate 115; a stack of the oxide semiconductor layers 130 a and 130 bin contact with the insulating layer 120; the conductive layers 140 and150 electrically connected to the stack; the oxide semiconductor layer130 c in contact with the stack and the conductive layers 140 and 150;the insulating layer 160 in contact with the oxide semiconductor layer130 c; the conductive layer 170 in contact with the insulating layer160; the insulating layer 175 in contact with the conductive layers 140and 150, the oxide semiconductor layer 130 c, the insulating layer 160,and the conductive layer 170; and the insulating layer 180 in contactwith the insulating layer 175. The insulating layer 180 may function asa planarization film as necessary.

The transistor 107 has the same structure as the transistor 101 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 231 and 232, thatthe oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 233, and that part ofthe oxide semiconductor layer (the oxide semiconductor layer 130 c)exists between the insulating layer 160 and the conductive layers 140and 150.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 32A and 32B. FIG. 32A is a top view of atransistor 108. A cross section in the direction of dashed-dotted line11-12 in FIG. 32A is illustrated in FIG. 32B. A cross section in thedirection of dashed-dotted line 13-14 in FIG. 32A is illustrated in FIG.37B. In some cases, the direction of dashed-dotted line 11-12 isreferred to as a channel length direction, and the direction ofdashed-dotted line 13-14 is referred to as a channel width direction.

The transistor 108 differs from the transistor 107 in that end portionsof the insulating layer 160 and the oxide semiconductor layer 130 c arenot aligned with the end portion of the conductive layer 170.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 33A and 33B. FIG. 33A is a top view of atransistor 109. A cross section in the direction of dashed-dotted lineJ1-J2 in FIG. 33A is illustrated in FIG. 33B. A cross section in thedirection of dashed-dotted line J3-J4 in FIG. 33A is illustrated in FIG.37A. In some cases, the direction of dashed-dotted line J1-J2 isreferred to as a channel length direction, and the direction ofdashed-dotted line J3-J4 is referred to as a channel width direction.

The transistor 109 includes the insulating layer 120 in contact with thesubstrate 115; a stack of the oxide semiconductor layers 130 a and 130 bin contact with the insulating layer 120; the oxide semiconductor layer130 c in contact with the stack; the insulating layer 160 in contactwith the oxide semiconductor layer 130 c; the conductive layer 170 incontact with the insulating layer 160; the insulating layer 175 coveringthe stack, the oxide semiconductor layer 130 c, the insulating layer160, and the conductive layer 170; the insulating layer 180 in contactwith the insulating layer 175; and the conductive layers 140 and 150electrically connected to the stack through openings provided in theinsulating layers 175 and 180. The transistor 109 may further include,for example, an insulating layer (planarization film) in contact withthe insulating layer 180 and the conductive layers 140 and 150 asnecessary.

The transistor 109 has the same structure as the transistor 103 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 231 and 232 andthat the oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 233.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 34A and 34B. FIG. 34A is a top view of atransistor 110. A cross section in the direction of dashed-dotted lineK1-K2 in FIG. 34A is illustrated in FIG. 34B. A cross section in thedirection of dashed-dotted line K3-K4 in FIG. 34A is illustrated in FIG.37A. In some cases, the direction of dashed-dotted line K1-K2 isreferred to as a channel length direction, and the direction ofdashed-dotted line K3-K4 is referred to as a channel width direction.

The transistor 110 has the same structure as the transistor 104 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 231 and 232 andthat the oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 233.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 35A and 35B. FIG. 35A is a top view of atransistor 111. A cross section in the direction of dashed-dotted lineL1-L2 in FIG. 35A is illustrated in FIG. 35B. A cross section in thedirection of dashed-dotted line L3-L4 in FIG. 35A is illustrated in FIG.37A. In some cases, the direction of dashed-dotted line L1-L2 isreferred to as a channel length direction, and the direction ofdashed-dotted line L3-L4 is referred to as a channel width direction.

The transistor 111 includes the insulating layer 120 in contact with thesubstrate 115; a stack of the oxide semiconductor layers 130 a and 130 bin contact with the insulating layer 120; the conductive layers 141 and151 electrically connected to the stack; the oxide semiconductor layer130 c in contact with the stack and the conductive layers 141 and 151;the insulating layer 160 in contact with the oxide semiconductor layer130 c; the conductive layer 170 in contact with the insulating layer160; the insulating layer 175 in contact with the stack, the conductivelayers 141 and 151, the oxide semiconductor layer 130 c, the insulatinglayer 160, and the conductive layer 170; the insulating layer 180 incontact with the insulating layer 175; and the conductive layers 142 and152 electrically connected to the conductive layers 141 and 151,respectively, through openings provided in the insulating layers 175 and180. The transistor 111 may further include, for example, an insulatinglayer (planarization film) in contact with the insulating layer 180 andthe conductive layers 142 and 152 as necessary.

The transistor 111 has the same structure as the transistor 105 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 231 and 232, thatthe oxide semiconductor layer 130 includes three layers (the oxidesemiconductor layers 130 a to 130 c) in the region 233, and that part ofthe oxide semiconductor layer (the oxide semiconductor layer 130 c)exists between the insulating layer 160 and the conductive layers 141and 151.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 36A and 36B. FIG. 36A is a top view of atransistor 112. A cross section in the direction of dashed-dotted lineM1-M2 in FIG. 36A is illustrated in FIG. 36B. A cross section in thedirection of dashed-dotted line M3-M4 in FIG. 36A is illustrated in FIG.37A. In some cases, the direction of dashed-dotted line M1-M2 isreferred to as a channel length direction, and the direction ofdashed-dotted line M3-M4 is referred to as a channel width direction.

The transistor 112 has the same structure as the transistor 106 exceptthat the oxide semiconductor layer 130 includes two layers (the oxidesemiconductor layers 130 a and 130 b) in the regions 331, 332, 334, and335 and that the oxide semiconductor layer 130 includes three layers(the oxide semiconductor layers 130 a to 130 c) in the region 333.

The transistor in one embodiment of the present invention may includethe conductive layer 173 between the oxide semiconductor layer 130 andthe substrate 115 as illustrated in cross-sectional views in the channellength direction in FIGS. 38A to 38F and cross-sectional views in thechannel width direction in FIGS. 37C and 37D. When the conductive layeris used as a second gate electrode layer (back gate), the on-statecurrent can be increased or the threshold voltage can be controlled. Inthe cross-sectional views in FIGS. 38A to 38F, the width of theconductive layer 173 may be shorter than that of the oxide semiconductorlayer 130. Moreover, the width of the conductive layer 173 may beshorter than that of the conductive layer 170.

As illustrated in FIG. 39A, the width (W_(SD)) of the conductive layer140 (source electrode layer) and the conductive layer 150 (drainelectrode layer) in the transistor in one embodiment of the presentinvention may be longer than the width (W_(OS)) of the oxidesemiconductor layer 130. Furthermore, as illustrated in FIG. 39B, W_(SD)may be shorter than W_(OS). When W_(OS)≧W_(SD) (W_(SD) is less than orequal to W_(OS)) is satisfied, a gate electric field is easily appliedto the entire oxide semiconductor layer 130, so that electricalcharacteristics of the transistor can be improved.

In the transistor in one embodiment of the present invention (any of thetransistors 101 to 112), the conductive layer 170 functioning as a gateelectrode layer electrically surrounds the oxide semiconductor layer 130in the channel width direction with the insulating layer 160 functioningas a gate insulating film positioned therebetween. This structureincreases the on-state current. Such a transistor structure is referredto as a surrounded channel (s-channel) structure.

In the transistor including the oxide semiconductor layers 130 a and 130b and the transistor including the oxide semiconductor layers 130 a to130 c, selecting appropriate materials for the two or three layersforming the oxide semiconductor layer 130 makes current flow to theoxide semiconductor layer 130 b. Since current flows to the oxidesemiconductor layer 130 b, the current is hardly influenced by interfacescattering, leading to high on-state current. Note that increasing thethickness of the oxide semiconductor layer 130 b can increase theon-state current. The thickness of the oxide semiconductor layer 130 bmay be, for example, 100 nm to 200 nm.

A semiconductor device including a transistor with any of the abovestructures can have favorable electrical characteristics.

Note that in this specification, the channel length refers to, forexample, a distance between a source (a source region or a sourceelectrode) and a drain (a drain region or a drain electrode) in a regionwhere a semiconductor (or a portion where current flows in asemiconductor when a transistor is on) and a gate electrode overlap witheach other or in a region where a channel is formed in a top view of thetransistor. In one transistor, channel lengths in all regions are notnecessarily the same. In other words, the channel length of onetransistor is not fixed to one value in some cases. Therefore, in thisspecification, the channel length is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductor(or a portion where current flows in a semiconductor when a transistoris on) and a gate electrode overlap with each other or a region where achannel is formed. In one transistor, channel widths in all regions donot necessarily have the same value. In other words, the channel widthof one transistor is not fixed to one value in some cases. Therefore, inthis specification, the channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is actually formed (hereinafter referred to as aneffective channel width) is sometimes different from a channel widthshown in a top view of a transistor (hereinafter referred to as anapparent channel width). For example, in a transistor having a gateelectrode covering a side surface of a semiconductor, an effectivechannel width is greater than an apparent channel width, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a gate electrode covering a side surfaceof a semiconductor, the proportion of a channel region formed in a sidesurface of a semiconductor is increased. In that case, an effectivechannel width is greater than an apparent channel width.

In such a case, measuring an effective channel width is difficult insome cases. For example, to estimate an effective channel width from adesign value, it is necessary to assume that the shape of asemiconductor is known. Therefore, in the case where the shape of asemiconductor is not known accurately, measuring an effective channelwidth accurately is difficult.

Accordingly, in this specification, an apparent channel width isreferred to as a surrounded channel width (SCW) in some cases.Furthermore, in this specification, the term “channel width” may denotea surrounded channel width, i.e., an apparent channel width or aneffective channel width. Note that the values of a channel length, achannel width, an effective channel width, an apparent channel width, asurrounded channel width, and the like can be determined by obtainingand analyzing a cross-sectional TEM image and the like.

A surrounded channel width may be used to calculate field-effectmobility, a current value per channel width, and the like of atransistor. In this case, the obtained value is sometimes different fromthe value obtained by using an effective channel width for thecalculation.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 5

In this embodiment, components of the transistors described inEmbodiment 4 are described in detail.

The substrate 115 includes a silicon substrate provided with atransistor and/or a photodiode; and an insulating layer, a wiring, aconductor functioning as a contact plug, and the like that are providedover the silicon substrate. Note that when p-ch transistors are formedusing the silicon substrate, a silicon substrate with n⁻-typeconductivity is preferably used. Alternatively, an SOI substrateincluding an n⁻-type or i-type silicon layer may be used. A surface ofthe silicon substrate where the transistor is formed preferably has a(110) plane orientation. Forming a p-ch transistor with the (110) planecan increase mobility.

The insulating layer 120 can have a function of supplying oxygen to theoxide semiconductor layer 130 as well as a function of preventingdiffusion of impurities from a component included in the substrate 115.For this reason, the insulating layer 120 is preferably an insulatingfilm containing oxygen and more preferably, the insulating layer 120 isan insulating film containing oxygen in which the oxygen content ishigher than that in the stoichiometric composition. The insulating layer120 is a film in which the amount of released oxygen when converted intooxygen atoms is preferably greater than or equal to 1.0×10¹⁹ atoms/cm³in TDS analysis. In the TDS analysis, the film surface temperature ishigher than or equal to 100° C. and lower than or equal to 700° C., orhigher than or equal to 100° C. and lower than or equal to 500° C. Theinsulating layer 120 also functions as an interlayer insulating film andmay be subjected to planarization treatment such as chemical mechanicalpolishing (CMP) so as to have a flat surface.

For example, the insulating layer 120 can be formed using an oxideinsulating film including aluminum oxide, magnesium oxide, siliconoxide, silicon oxynitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,tantalum oxide, or the like; a nitride insulating film including siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like; or a mixed material of any of these. The insulatinglayer 120 may be a stack of any of the above materials.

In this embodiment, detailed description is given mainly on the casewhere the oxide semiconductor layer 130 of the transistor has athree-layer structure in which the oxide semiconductor layers 130 a to130 c are sequentially stacked from the insulating layer 120 side.

Note that in the case where the oxide semiconductor layer 130 is asingle layer, a layer corresponding to the oxide semiconductor layer 130b described in this embodiment is used.

In the case where the oxide semiconductor layer 130 has a two-layerstructure, a stack in which a layer corresponding to the oxidesemiconductor layer 130 a and a layer corresponding to the oxidesemiconductor layer 130 b are sequentially stacked from the insulatinglayer 120 side described in this embodiment is used. In such a case, theoxide semiconductor layers 130 a and 130 b can be replaced with eachother.

In the case where the oxide semiconductor layer 130 has a layeredstructure of four or more layers, for example, a structure in whichanother oxide semiconductor layer is added to the three-layer stack ofthe oxide semiconductor layer 130 described in this embodiment can beemployed.

For the oxide semiconductor layer 130 b, for example, an oxidesemiconductor whose electron affinity (an energy difference between avacuum level and the conduction band minimum) is higher than those ofthe oxide semiconductor layers 130 a and 130 c is used. The electronaffinity can be obtained by subtracting an energy difference between theconduction band minimum and the valence band maximum (what is called anenergy gap) from an energy difference between the vacuum level and thevalence band maximum (what is called an ionization potential).

The oxide semiconductor layers 130 a and 130 c each contain one or morekinds of metal elements contained in the oxide semiconductor layer 130b. For example, the oxide semiconductor layers 130 a and 130 c arepreferably formed using an oxide semiconductor whose conduction bandminimum is closer to a vacuum level than that of the oxide semiconductorlayer 130 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV orless.

In such a structure, when an electric field is applied to the conductivelayer 170, a channel is formed in the oxide semiconductor layer 130 bwhose conduction band minimum is the lowest in the oxide semiconductorlayer 130.

Furthermore, since the oxide semiconductor layer 130 a contains one ormore kinds of metal elements contained in the oxide semiconductor layer130 b, an interface state is unlikely to be formed at the interfacebetween the oxide semiconductor layers 130 a and 130 b, compared withthe interface between the oxide semiconductor layer 130 b and theinsulating layer 120 on the assumption that the oxide semiconductorlayer 130 b is in contact with the insulating layer 120. The interfacestate sometimes forms a channel; therefore, the threshold voltage of thetransistor is changed in some cases. Thus, with the oxide semiconductorlayer 130 a, variations in electrical characteristics of the transistor,such as a threshold voltage, can be reduced. Moreover, the reliabilityof the transistor can be improved.

Furthermore, since the oxide semiconductor layer 130 c contains one ormore kinds of metal elements contained in the oxide semiconductor layer130 b, scattering of carriers is unlikely to occur at the interfacebetween the oxide semiconductor layers 130 b and 130 c, compared withthe interface between the oxide semiconductor layer 130 b and the gateinsulating film (the insulating layer 160) on the assumption that theoxide semiconductor layer 130 b is in contact with the gate insulatingfilm. Thus, with the oxide semiconductor layer 130 c, the field-effectmobility of the transistor can be increased.

For the oxide semiconductor layers 130 a and 130 c, for example, amaterial containing Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf with ahigher atomic ratio than that used for the oxide semiconductor layer 130b can be used. Specifically, the atomic ratio of any of the above metalelements in the oxide semiconductor layers 130 a and 130 c is 1.5 timesor more, preferably 2 times or more, more preferably 3 times or more aslarge as that in the oxide semiconductor layer 130 b. Any of the abovemetal elements is strongly bonded to oxygen and thus has a function ofsuppressing generation of an oxygen vacancy in the oxide semiconductorlayers 130 a and 130 c. That is, an oxygen vacancy is less likely to begenerated in the oxide semiconductor layers 130 a and 130 c than in theoxide semiconductor layer 130 b.

An oxide semiconductor that can be used for each of the oxidesemiconductor layers 130 a to 130 c preferably contains at least In orZn. Both In and Zn are preferably contained. In order to reducevariations in electrical characteristics of the transistor including theoxide semiconductor, the oxide semiconductor preferably contains astabilizer in addition to In and Zn.

Examples of a stabilizer include Ga, Sn, Hf, Al, and Zr. Other examplesof the stabilizer include lanthanoids such as La, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

As the oxide semiconductor, for example, any of the following can beused: indium oxide, tin oxide, gallium oxide, zinc oxide, an In—Znoxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, anIn—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, anIn—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide,an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Znoxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, anIn—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide,an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Znoxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide,an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

For example, an In—Ga—Zn oxide means an oxide containing In, Ga, and Znas its main components. The In—Ga—Zn oxide may contain another metalelement in addition to In, Ga, and Zn. In this specification, a filmcontaining the In—Ga—Zn oxide is also referred to as an IGZO film.

A material represented by InMO₃(ZnO)_(m) (m>0, where m is not aninteger) may be used. Note that M represents one or more metal elementsselected from Ga, Y, Zr, La, Ce, and Nd. Alternatively, a materialrepresented by In₂SnO₅(ZnO)_(n) (n>0, where n is an integer) may beused.

Note that when each of the oxide semiconductor layers 130 a to 130 c isan In—M—Zn oxide containing at least indium, zinc, and M (M is a metalsuch as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), in the case where theoxide semiconductor layer 130 a has an atomic ratio of In to M and Znwhich is x₁:y₁:z₁, the oxide semiconductor layer 130 b has an atomicratio of In to M and Zn which is x₂:y₂:z₂, and the oxide semiconductorlayer 130 c has an atomic ratio of In to M and Zn which is x₃:y₃:z₃,each of y₁/x₁ and y₃/x₃ is preferably larger than y₂/x₂. Each of y₁/x₁and y₃/x₃ is 1.5 times or more, preferably 2 times or more, morepreferably 3 times or more as large as y₂/x₂. At this time, when y₂ isgreater than or equal to x₂ in the oxide semiconductor layer 130 b, thetransistor can have stable electrical characteristics. However, when y₂is 3 times or more as large as x₂, the field-effect mobility of thetransistor is reduced; accordingly, y₂ is preferably smaller than 3times x₂.

In the case where Zn and O are not taken into consideration, theproportion of In and the proportion of M in each of the oxidesemiconductor layers 130 a and 130 c are preferably less than 50 atomic% and greater than or equal to 50 atomic %, respectively, morepreferably less than 25 atomic % and greater than or equal to 75 atomic%, respectively. Furthermore, in the case where Zn and O are not takeninto consideration, the proportion of In and the proportion of M in theoxide semiconductor layer 130 b are preferably greater than or equal to25 atomic % and less than 75 atomic %, respectively, more preferablygreater than or equal to 34 atomic % and less than 66 atomic %,respectively.

The indium content in the oxide semiconductor layer 130 b is preferablyhigher than those in the oxide semiconductor layers 130 a and 130 c. Inan oxide semiconductor, the s orbital of heavy metal mainly contributesto carrier transfer, and when the proportion of In in the oxidesemiconductor is increased, overlap of the s orbitals is likely to beincreased. Therefore, an oxide in which the proportion of In is higherthan that of M has higher mobility than an oxide in which the proportionof In is equal to or lower than that of M. Thus, with the use of anoxide having a high content of indium for the oxide semiconductor layer130 b, a transistor having high field-effect mobility can be obtained.

The thickness of the oxide semiconductor layer 130 a is greater than orequal to 3 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 50 nm, more preferablygreater than or equal to 5 nm and less than or equal to 25 nm. Thethickness of the oxide semiconductor layer 130 b is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 10 nm and less than or equal to 150 nm, more preferablygreater than or equal to 15 nm and less than or equal to 100 nm. Thethickness of the oxide semiconductor layer 130 c is greater than orequal to 1 nm and less than or equal to 50 nm, preferably greater thanor equal to 2 nm and less than or equal to 30 nm, more preferablygreater than or equal to 3 nm and less than or equal to 15 nm. Inaddition, the oxide semiconductor layer 130 b is preferably thicker thanthe oxide semiconductor layers 130 a and 130 c.

Note that in order that a transistor in which a channel is formed in anoxide semiconductor layer have stable electrical characteristics, it iseffective to make the oxide semiconductor layer intrinsic orsubstantially intrinsic by reducing the concentration of impurities inthe oxide semiconductor layer. The term “substantially intrinsic” refersto a state where an oxide semiconductor layer has a carrier densitylower than 1×10¹⁷/cm³, lower than 1×10¹⁵/cm³, or lower than 1×10¹³/cm³.

In the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon,and a metal element other than main components of the oxidesemiconductor layer are impurities. For example, hydrogen and nitrogenform donor levels to increase the carrier density, and silicon formsimpurity levels in the oxide semiconductor layer. The impurity levelsserve as traps and might cause deterioration of electricalcharacteristics of the transistor. Therefore, it is preferable to reducethe concentration of the impurities in the oxide semiconductor layers130 a to 130 c and at interfaces between the oxide semiconductor layers.

In order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, the oxide semiconductor layer is controlled tohave a region in which the concentration of hydrogen, which is measuredby secondary ion mass spectrometry (SIMS), is lower than or equal to2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³,more preferably lower than or equal to 1×10¹⁹ atoms/cm³, still morepreferably lower than or equal to 5×10¹⁸ atoms/cm³ and is higher than orequal to 1×10¹⁷ atoms/cm³. Furthermore, the concentration of nitrogen islower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, more preferably lower than or equal to 1×10¹⁸ atoms/cm³,still more preferably lower than or equal to 5×10¹⁷ atoms/cm³ and ishigher than or equal to 5×10¹⁶ atoms/cm³.

The high concentration of silicon or carbon might reduce thecrystallinity of the oxide semiconductor layer. In order not to lowerthe crystallinity of the oxide semiconductor layer, for example, theoxide semiconductor layer is controlled to have a region in which theconcentration of silicon is lower than 1×10¹⁹ atoms/cm³, preferablylower than 5×10¹⁸ atoms/cm³, more preferably lower than 1×10¹⁸ atoms/cm³and is higher than or equal to 1×10¹⁸ atoms/cm³. Furthermore, the oxidesemiconductor layer is controlled to have a region in which theconcentration of carbon is lower than 1×10¹⁹ atoms/cm³, preferably lowerthan 5×10¹⁸ atoms/cm³, more preferably lower than 1×10¹⁸ atoms/cm³ andis higher than or equal to 6×10¹⁷ atoms/cm³.

A transistor in which a highly purified oxide semiconductor film is usedfor a channel formation region exhibits extremely low off-state current.When voltage between a source and a drain is set at about 0.1 V, 5 V, or10 V, for example, the off-state current per channel width of thetransistor can be as low as several yoctoamperes per micrometer toseveral zeptoamperes per micrometer.

As the gate insulating film of the transistor, an insulating filmcontaining silicon is used in many cases; thus, it is preferable that,as in the transistor in one embodiment of the present invention, aregion of the oxide semiconductor layer that serves as a channel not bein contact with the gate insulating film for the above reason. In thecase where a channel is formed at the interface between the gateinsulating film and the oxide semiconductor layer, scattering ofcarriers occurs at the interface, so that the field-effect mobility ofthe transistor is reduced. Also from the view of the above, it ispreferable that the region of the oxide semiconductor layer that servesas a channel be separated from the gate insulating film.

Accordingly, with the oxide semiconductor layer 130 having a layeredstructure including the oxide semiconductor layers 130 a to 130 c, achannel can be formed in the oxide semiconductor layer 130 b; thus, thetransistor can have high field-effect mobility and stable electricalcharacteristics.

In a band structure, the conduction band minimums of the oxidesemiconductor layers 130 a to 130 c are continuous. This can beunderstood also from the fact that the compositions of the oxidesemiconductor layers 130 a to 130 c are close to one another and oxygenis easily diffused among the oxide semiconductor layers 130 a to 130 c.Thus, the oxide semiconductor layers 130 a to 130 c have a continuousphysical property though they have different compositions and form astack. In the drawings, interfaces between the oxide semiconductorlayers of the stack are indicated by dotted lines.

The oxide semiconductor layer 130 in which layers containing the samemain components are stacked is formed to have not only a simple layeredstructure of the layers but also a continuous energy band (here, inparticular, a well structure having a U shape in which the conductionband minimums are continuous (U-shape well)). In other words, thelayered structure is formed such that there exists no impurity thatforms a defect level such as a trap center or a recombination center ateach interface. If impurities exist between the stacked oxidesemiconductor layers, the continuity of the energy band is lost andcarriers disappear by a trap or recombination at the interface.

For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:4:5, 1:6:4, or 1:9:6 can be used for theoxide semiconductor layers 130 a and 130 c, and an In—Ga—Zn oxide whoseatomic ratio of In to Ga and Zn is 1:1:1, 2:1:3, 5:5:6, or 3:1:2 can beused for the oxide semiconductor layer 130 b. In each of the oxidesemiconductor layers 130 a to 130 c, the proportion of each atom in theatomic ratio varies within a range of ±20% as a margin.

The oxide semiconductor layer 130 b of the oxide semiconductor layer 130serves as a well, so that a channel is formed in the oxide semiconductorlayer 130 b. Note that since the conduction band minimums arecontinuous, the oxide semiconductor layer 130 can also be referred to asa U-shaped well. Furthermore, a channel formed to have such a structurecan also be referred to as a buried channel.

Note that trap levels due to impurities or defects might be formed inthe vicinity of the interface between an insulating layer such as asilicon oxide film and each of the oxide semiconductor layers 130 a and130 c. The oxide semiconductor layer 130 b can be distanced away fromthe trap levels owing to existence of the oxide semiconductor layers 130a and 130 c.

However, when the energy differences between the conduction band minimumof the oxide semiconductor layer 130 b and the conduction band minimumof each of the oxide semiconductor layers 130 a and 130 c are small, anelectron in the oxide semiconductor layer 130 b might reach the traplevel by passing over the energy differences. When the electron istrapped in the trap level, negative charge is generated at the interfacewith the insulating layer, so that the threshold voltage of thetransistor is shifted in a positive direction.

The oxide semiconductor layers 130 a to 130 c preferably include crystalparts. In particular, when crystals with c-axis alignment are used, thetransistor can have stable electrical characteristics. Moreover,crystals with c-axis alignment are resistant to bending; therefore,using such crystals can improve the reliability of a semiconductordevice using a flexible substrate.

As the conductive layer 140 functioning as a source electrode layer andthe conductive layer 150 functioning as a drain electrode layer, forexample, a single layer or a stacked layer formed using a materialselected from Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and alloysof any of these metal materials can be used. Typically, it is preferableto use Ti, which is particularly easily bonded to oxygen, or W, whichhas a high melting point and thus makes subsequent process temperaturescomparatively high. It is also possible to use a stack of any of theabove materials and Cu or an alloy such as Cu—Mn, which has lowresistance. In the transistors 105, 106, 111, and 112, for example, itis possible to use W for the conductive layers 141 and 151 and use astack of Ti and Al for the conductive layers 142 and 152.

The above materials are capable of extracting oxygen from an oxidesemiconductor layer. Therefore, in a region of the oxide semiconductorlayer that is in contact with any of the above materials, oxygen isreleased from the oxide semiconductor layer and an oxygen vacancy isformed. Hydrogen slightly contained in the layer and the oxygen vacancyare bonded to each other, so that the region is changed to an n-typeregion. Accordingly, the n-type region can serve as a source or a drainof the transistor.

In the case where W is used for the conductive layers 140 and 150, theconductive layers 140 and 150 may be doped with nitrogen. Doping withnitrogen can appropriately lower the capability of extracting oxygen andprevent the n-type region from spreading to a channel region. It ispossible to prevent the n-type region from spreading to a channel regionalso by using a stack of W and an n-type semiconductor layer as theconductive layers 140 and 150 and putting the n-type semiconductor layerin contact with the oxide semiconductor layer. As the n-typesemiconductor layer, an In—Ga—Zn oxide, zinc oxide, indium oxide, tinoxide, indium tin oxide, or the like to which nitrogen is added can beused.

The insulating layer 160 functioning as a gate insulating film can beformed using an insulating film containing one or more of aluminumoxide, magnesium oxide, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,and tantalum oxide. The insulating layer 160 may be a stack includingany of the above materials. The insulating layer 160 may contain La, N,Zr, or the like as an impurity.

An example of a layered structure of the insulating layer 160 isdescribed. The insulating layer 160 includes, for example, oxygen,nitrogen, silicon, or hafnium. Specifically, the insulating layer 160preferably includes hafnium oxide and silicon oxide or siliconoxynitride.

Hafnium oxide and aluminum oxide have higher dielectric constants thansilicon oxide and silicon oxynitride. Therefore, the insulating layer160 using hafnium oxide or aluminum oxide can have larger thickness thanthe insulating layer 160 using silicon oxide, so that leakage currentdue to tunnel current can be reduced. That is, a transistor with lowoff-state current can be provided. Moreover, hafnium oxide with acrystalline structure has a higher dielectric constant than hafniumoxide with an amorphous structure. Therefore, it is preferable to usehafnium oxide with a crystalline structure in order to provide atransistor with low off-state current. Examples of the crystal structureinclude a monoclinic crystal structure and a cubic crystal structure.Note that one embodiment of the present invention is not limited to theabove examples.

For the insulating layers 120 and 160 in contact with the oxidesemiconductor layer 130, a film that releases less nitrogen oxide ispreferably used. For the insulating layers 120 and 160, for example, asilicon oxynitride film or an aluminum oxynitride film that releasesless nitrogen oxide can be used.

A silicon oxynitride film that releases less nitrogen oxide is a film ofwhich the amount of released ammonia is larger than the amount ofreleased nitrogen oxide in TDS; the amount of released ammonia istypically greater than or equal to 1×10¹⁸ molecules/cm³ and less than orequal to 5×10¹⁹ molecules/cm³. Note that the amount of released ammoniais the amount of ammonia released by heat treatment with which thesurface temperature of the film becomes higher than or equal to 50° C.and lower than or equal to 650° C., preferably higher than or equal to50° C. and lower than or equal to 550° C.

By using the above oxide insulating layer for the insulating layers 120and 160, a shift in the threshold voltage of the transistor can bereduced, which leads to reduced fluctuations in the electricalcharacteristics of the transistor.

For the conductive layer 170 functioning as a gate electrode layer, forexample, a conductive film formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr,Mo, Ru, Ag, Mn, Nd, Sc, Ta, W, or the like can be used. Alternatively,an alloy or a conductive nitride of any of these materials may be used.Alternatively, a stack of a plurality of materials selected from thesematerials, alloys of these materials, and conductive nitrides of thesematerials may be used. Typically, tungsten, a stack of tungsten andtitanium nitride, a stack of tungsten and tantalum nitride, or the likecan be used. Alternatively, Cu or an alloy such as Cu—Mn, which has lowresistance, or a stack of any of the above materials and Cu or an alloysuch as Cu—Mn may be used. In this embodiment, tantalum nitride is usedfor the conductive layer 171 and tungsten is used for the conductivelayer 172 to form the conductive layer 170.

As the insulating layer 175, a silicon nitride film, an aluminum nitridefilm, or the like containing hydrogen can be used. In the transistors103, 104, 106, 109, 110, and 112 described in Embodiment 4, when aninsulating film containing hydrogen is used as the insulating layer 175,part of the oxide semiconductor layer can have n-type conductivity. Inaddition, a nitride insulating film functions as a blocking film againstmoisture and the like and can improve the reliability of the transistor.

An aluminum oxide film can also be used as the insulating layer 175. Itis particularly preferable to use an aluminum oxide film as theinsulating layer 175 in the transistors 101, 102, 105, 107, 108, and 111described in Embodiment 4. The aluminum oxide film has a high blockingeffect of preventing penetration of both oxygen and impurities such ashydrogen and moisture. Accordingly, during and after the manufacturingprocess of the transistor, the aluminum oxide film can suitably functionas a protective film that has effects of preventing entry of impuritiessuch as hydrogen and moisture into the oxide semiconductor layer 130,preventing release of oxygen from the oxide semiconductor layer, andpreventing unnecessary release of oxygen from the insulating layer 120.Furthermore, oxygen contained in the aluminum oxide film can be diffusedinto the oxide semiconductor layer.

Furthermore, the insulating layer 180 is preferably formed over theinsulating layer 175. The insulating layer 180 can be formed using aninsulating film containing one or more of magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating layer 180 may be a stack of any of the above materials.

Here, like the insulating layer 120, the insulating layer 180 preferablycontains oxygen more than that in the stoichiometric composition. Oxygenreleased from the insulating layer 180 can be diffused into the channelformation region in the oxide semiconductor layer 130 through theinsulating layer 160, so that oxygen vacancies formed in the channelformation region can be filled with oxygen. In this manner, stableelectrical characteristics of the transistor can be achieved.

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration of electrical characteristics of the transistor. Inparticular, a decrease in channel width causes a reduction in on-statecurrent.

In the transistors 107 to 112 in one embodiment of the presentinvention, the oxide semiconductor layer 130 c is formed to cover theoxide semiconductor layer 130 b where a channel is formed; thus, achannel formation layer is not in contact with the gate insulating film.Accordingly, scattering of carriers at the interface between the channelformation layer and the gate insulating film can be reduced and theon-state current of the transistor can be increased.

In the transistor in one embodiment of the present invention, asdescribed above, the gate electrode layer (the conductive layer 170) isformed to electrically surround the oxide semiconductor layer 130 in thechannel width direction; accordingly, a gate electric field is appliedto the oxide semiconductor layer 130 in a direction perpendicular to itsside surface in addition to a direction perpendicular to its topsurface. In other words, a gate electric field is applied to the entirechannel formation layer and effective channel width is increased,leading to a further increase in the on-state current.

Furthermore, in the transistor in one embodiment of the presentinvention in which the oxide semiconductor layer 130 has a two-layerstructure or a three-layer structure, since the oxide semiconductorlayer 130 b where a channel is formed is provided over the oxidesemiconductor layer 130 a, an effect of making an interface state lesslikely to be formed is obtained. In the transistor in one embodiment ofthe present invention in which the oxide semiconductor layer 130 has athree-layer structure, since the oxide semiconductor layer 130 b ispositioned at the middle of the three-layer structure, an effect ofeliminating the influence of an impurity that enters from upper andlower layers on the oxide semiconductor layer 130 b is obtained as well.Therefore, the transistor can achieve not only the increase in theon-state current of the transistor but also stabilization of thethreshold voltage and a reduction in the S value (subthreshold value).Thus, current when gate voltage VG is 0 V can be reduced and powerconsumption can be reduced. In addition, since the threshold voltage ofthe transistor becomes stable, long-term reliability of thesemiconductor device can be improved. Furthermore, the transistor in oneembodiment of the present invention is suitable for a highly integratedsemiconductor device because deterioration of electrical characteristicsdue to miniaturization is reduced.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 6

In this embodiment, methods for manufacturing the transistors 101, 107,and 111 described in Embodiment 4 are described.

First, a method for manufacturing a silicon transistor included in thesubstrate 115 is described. Here, an example of a method formanufacturing a p-ch transistor is described. An n⁻-type single crystalsilicon substrate is used as a silicon substrate, and an elementformation region isolated with an insulating layer (also referred to asa field oxide film) is formed in the surface. The element formationregion can be formed by local oxidation of silicon (LOCOS), shallowtrench isolation (STI), or the like.

Here, the substrate is not limited to the single crystal siliconsubstrate. A silicon on insulator (SOI) substrate or the like can alsobe used.

Next, a gate insulating film is formed to cover the element formationregion. For example, a silicon oxide film is formed by oxidation of asurface of the element formation region by heat treatment. Furthermore,after the silicon oxide film is formed, a surface of the silicon oxidefilm may be nitrided by nitriding treatment.

Next, a conductive film is formed to cover the gate insulating film. Theconductive film can be formed using an element selected from Ta, W, Ti,Mo, Al, Cu, Cr, Nb, and the like, or an alloy material or a compoundmaterial containing such an element as a main component. Alternatively,a metal nitride film obtained by nitriding of any of these elements canbe used. Alternatively, a semiconductor material typified bypolycrystalline silicon doped with an impurity element such asphosphorus can be used.

Then, the conductive film is selectively etched, so that a gateelectrode layer is formed over the gate insulating film.

Next, an insulating film such as a silicon oxide film or a siliconnitride film is formed to cover the gate electrode layer and etch backis performed, so that sidewalls are formed on side surfaces of the gateelectrode layer.

Next, a resist mask is selectively formed to cover regions except theelement formation region, and an impurity element is added using theresist mask and the gate electrode layer as masks, so that pt-typeimpurity regions are formed. Here, in order to form a p-ch transistor,an impurity element imparting p-type conductivity such as B or Ga can beused as the impurity element.

Through the above steps, a p-ch transistor including an active region inthe silicon substrate is completed. Note that a passivation film such asa silicon nitride film or an aluminum oxide film is preferably formedover the transistor.

Next, an interlayer insulating film is formed over the silicon substratewhere the transistor is formed, and contact plugs and wirings areformed.

A method for manufacturing the transistor 101 is described withreference to FIGS. 40A to 40C and FIGS. 41A to 41C. A cross section ofthe transistor in the channel length direction is shown on the leftside, and a cross section of the transistor in the channel widthdirection is shown on the right side. The cross-sectional views in thechannel width direction are enlarged views; therefore, components on theleft side and those on the right side differ in apparent thickness.

The case where the oxide semiconductor layer 130 has a three-layerstructure of the oxide semiconductor layers 130 a to 130 c is describedas an example. In the case where the oxide semiconductor layer 130 has atwo-layer structure, the oxide semiconductor layers 130 a and 130 b areused. In the case where the oxide semiconductor layer 130 has asingle-layer structure, the oxide semiconductor layer 130 b is used.

First, the insulating layer 120 is formed over the substrate 115.Embodiment 5 can be referred to for the kind of the substrate 115 andthe material of the insulating layer 120. The insulating layer 120 canbe formed by sputtering, CVD, molecular beam epitaxy (MBE), or the like.

Oxygen may be added to the insulating layer 120 by ion implantation, iondoping, plasma immersion ion implantation, plasma treatment, or thelike. Adding oxygen enables the insulating layer 120 to supply oxygenmuch easily to the oxide semiconductor layer 130.

In the case where a surface of the substrate 115 is made of an insulatorand there is no influence of impurity diffusion on the oxidesemiconductor layer 130 to be formed later, the insulating layer 120 isnot necessarily provided.

Next, an oxide semiconductor film 130A to be the oxide semiconductorlayer 130 a, an oxide semiconductor film 130B to be the oxidesemiconductor layer 130 b, and an oxide semiconductor film 130C to bethe oxide semiconductor layer 130 c are formed over the insulating layer120 by sputtering, CVD, MBE, or the like (see FIG. 40A).

In the case where the oxide semiconductor layer 130 has a layeredstructure, oxide semiconductor films are preferably formed successivelywithout exposure to the air with the use of a multi-chamber depositionapparatus (e.g., a sputtering apparatus) including a load lock chamber.It is preferable that each chamber of the sputtering apparatus be ableto be evacuated to a high vacuum (approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa)by an adsorption vacuum evacuation pump such as a cryopump and that thechamber be able to heat the substrate to higher than or equal to 100°C., preferably higher than or equal to 500° C., so that water and thelike serving as impurities of an oxide semiconductor are removed as muchas possible. Alternatively, the combination of a turbo molecular pumpand a cold trap is preferably used to prevent back-flow of a gascontaining a carbon component, moisture, or the like from an exhaustsystem into the chamber. Alternatively, the combination of a turbomolecular pump and a cryopump may be used as an exhaust system.

Not only high vacuum evacuation of the chamber but also high purity of asputtering gas is preferred to obtain a highly purified intrinsic oxidesemiconductor. As an oxygen gas or an argon gas used for a sputteringgas, a gas which is highly purified to have a dew point of −40° C. orlower, preferably −80° C. or lower, more preferably −100° C. or lower isused, so that entry of moisture or the like into the oxide semiconductorfilm can be prevented as much as possible.

For the oxide semiconductor films 130A to 130C, any of the materialsdescribed in Embodiment 5 can be used. In the case where sputtering isused for deposition, any of the materials described in Embodiment 5 canbe used as a target.

Note that as described in detail in Embodiment 5, a material that has ahigher electron affinity than the oxide semiconductor films 130A and130C is used for the oxide semiconductor film 130B.

The oxide semiconductor films are preferably formed by sputtering. Assputtering, RF sputtering, DC sputtering, AC sputtering, or the like canbe used.

After the oxide semiconductor film 130C is formed, first heat treatmentmay be performed. The first heat treatment may be performed at atemperature higher than or equal to 250° C. and lower than or equal to650° C., preferably higher than or equal to 300° C. and lower than orequal to 500° C., in an inert gas atmosphere, an atmosphere containingan oxidizing gas at 10 ppm or more, or a reduced pressure state.Alternatively, the first heat treatment may be performed in such amanner that heat treatment is performed in an inert gas atmosphere, andthen another heat treatment is performed in an atmosphere containing anoxidizing gas at 10 ppm or more, in order to compensate released oxygen.The first heat treatment can increase the crystallinity of the oxidesemiconductor films 130A to 130C and remove impurities such as water andhydrogen from the insulating layer 120 and the oxide semiconductor films130A to 130C. Note that the first heat treatment may be performed afteretching for forming the oxide semiconductor layers 130 a to 130 cdescribed later.

Next, a conductive layer is formed over the oxide semiconductor film130C. The conductive layer can be, for example, formed by the followingmethod.

First, a first conductive film is formed over the oxide semiconductorfilm 130C. As the first conductive film, a single layer or a stackedlayer can be formed using a material selected from Al, Cr, Cu, Ta, Ti,Mo, W, Ni, Mn, Nd, and Sc and an alloy of any of these metal materials.

Next, a resist film is formed over the first conductive film and theresist film is exposed to light by electron beam exposure, liquidimmersion exposure, or EUV exposure and developed, so that a firstresist mask is formed. An organic coating film is preferably formed asan adherence agent between the first conductive film and the resistfilm. Alternatively, the first resist mask may be formed by nanoimprintlithography.

Then, the first conductive film is selectively etched using the firstresist mask and the first resist mask is subjected to ashing; thus, theconductive layer is formed.

Next, the oxide semiconductor films 130A to 130C are selectively etchedusing the conductive layer as a hard mask and the conductive layer isremoved; thus, the oxide semiconductor layer 130 including a stack ofthe oxide semiconductor layers 130 a to 130 c is formed (see FIG. 40B).It is also possible to form the oxide semiconductor layer 130 using thefirst resist mask, without forming the conductive layer. Here, oxygenions may be implanted into the oxide semiconductor layer 130.

Next, a second conductive film is formed to cover the oxidesemiconductor layer 130. The second conductive film can be formed usinga material that can be used for the conductive layers 140 and 150described in Embodiment 5. Sputtering, CVD, MBE, or the like can be usedfor the formation of the second conductive film.

Then, a second resist mask is formed over portions to be a source regionand a drain region. Then, part of the second conductive film is etched,so that the conductive layers 140 and 150 are formed (see FIG. 40C).

Next, an insulating film 160A is formed over the oxide semiconductorlayer 130 and the conductive layers 140 and 150. The insulating film160A can be formed using a material that can be used for the insulatinglayer 160 described in Embodiment 5. Sputtering, CVD, MBE, or the likecan be used for the formation of the insulating film 160A.

After that, second heat treatment may be performed. The second heattreatment can be performed in a condition similar to that of the firstheat treatment. The second heat treatment can make oxygen diffuse fromthe insulating layer 120 into the entire oxide semiconductor layer 130.Note that it is possible to obtain this effect by third heat treatment,without performing the second heat treatment.

Then, a third conductive film 171A and a fourth conductive film 172A tobe the conductive layer 170 are formed over the insulating film 160A.The third conductive film 171A and the fourth conductive film 172A canbe formed using materials that can be used for the conductive layers 171and 172 described in Embodiment 5. Sputtering, CVD, MBE, or the like canbe used for the formation of the third conductive film 171A and thefourth conductive film 172A.

Next, a third resist mask 156 is formed over the fourth conductive film172A (see FIG. 41A). The third conductive film 171A, the fourthconductive film 172A, and the insulating film 160A are selectivelyetched using the third resist mask 156, so that the conductive layer 170including the conductive layers 171 and 172 and the insulating layer 160are formed (see FIG. 41B). Note that if the insulating film 160A is notetched, the transistor 102 can be manufactured.

After that, the insulating layer 175 is formed over the oxidesemiconductor layer 130, the conductive layers 140 and 150, theinsulating layer 160, and the conductive layer 170. Embodiment 5 can bereferred to for the material of the insulating layer 175. In thetransistor 101, an aluminum oxide film is preferably used. Theinsulating layer 175 can be formed by sputtering, CVD, MBE, or the like.

Next, the insulating layer 180 is formed over the insulating layer 175(see FIG. 41C). Embodiment 5 can be referred to for the material of theinsulating layer 180. The insulating layer 180 can be formed bysputtering, CVD, MBE, or the like.

Oxygen may be added to the insulating layer 175 and/or the insulatinglayer 180 by ion implantation, ion doping, plasma immersion ionimplantation, plasma treatment, or the like. Adding oxygen enables theinsulating layer 175 and/or the insulating layer 180 to supply oxygenmuch easily to the oxide semiconductor layer 130.

Next, the third heat treatment may be performed. The third heattreatment can be performed in a condition similar to that of the firstheat treatment. By the third heat treatment, excess oxygen is easilyreleased from the insulating layers 120, 175, and 180, so that oxygenvacancies in the oxide semiconductor layer 130 can be reduced.

Next, a method for manufacturing the transistor 107 is described. Notethat detailed description of steps similar to those for manufacturingthe transistor 102 described above is omitted.

The insulating layer 120 is formed over the substrate 115, and the oxidesemiconductor film 130A to be the oxide semiconductor layer 130 a andthe oxide semiconductor film 130B to be the oxide semiconductor layer130 b are formed over the insulating layer 120 by sputtering, CVD, MBE,or the like (see FIG. 42A).

After that, a first conductive film is formed over the oxidesemiconductor film 130B, and a conductive layer is formed using a firstresist mask by a method similar to the above method. Then, the oxidesemiconductor films 130A and 130B are selectively etched using theconductive layer as a hard mask and the conductive layer is removed;thus, a stack of the oxide semiconductor layers 130 a and 130 b isformed (see FIG. 42B). It is also possible to form the stack using thefirst resist mask, without forming the hard mask. Here, oxygen ions maybe implanted into the oxide semiconductor layers 130 a and 130 b.

Next, a second conductive film is formed to cover the stack. Then, asecond resist mask is formed over portions to be a source region and adrain region, and part of the second conductive film is etched using thesecond resist mask, so that the conductive layers 140 and 150 are formed(see FIG. 42C).

After that, the oxide semiconductor film 130C to be the oxidesemiconductor layer 130 c is formed over the stack of the oxidesemiconductor layers 130 a and 130 b and the conductive layers 140 and150. Furthermore, the insulating film 160A, the third conductive film171A, and the fourth conductive film 172A are formed over the oxidesemiconductor film 130C.

Then, the third resist mask 156 is formed over the fourth conductivefilm 172A (see FIG. 43A). The third conductive film 171A, the fourthconductive film 172A, the insulating film 160A, and the oxidesemiconductor film 130C are selectively etched using the resist mask, sothat the conductive layer 170 including the conductive layers 171 and172, the insulating layer 160, and the oxide semiconductor layer 130 care formed (see FIG. 43B). Note that when the insulating film 160A andthe oxide semiconductor film 130C are etched using a fourth resist mask,the transistor 108 can be manufactured.

Next, the insulating layers 175 and 180 are formed over the insulatinglayer 120, the oxide semiconductor layer 130 (the oxide semiconductorlayers 130 a to 130 c), the conductive layers 140 and 150, theinsulating layer 160, and the conductive layer 170 (see FIG. 43C).

Through the above steps, the transistor 107 can be manufactured.

Next, a method for manufacturing the transistor 111 is described. Notethat detailed description of steps similar to those for manufacturingthe transistor 102 described above is omitted.

The insulating layer 120 is formed over the substrate 115, and the oxidesemiconductor film 130A to be the oxide semiconductor layer 130 a andthe oxide semiconductor film 130B to be the oxide semiconductor layer130 b are formed over the insulating layer 120 by sputtering, CVD, MBE,or the like. Then, a first conductive film is formed over the oxidesemiconductor film 130B, and a conductive layer 141 a is formed using afirst resist mask (see FIG. 44A).

Then, the oxide semiconductor films 130A and 130B are selectively etchedusing the conductive layer 141 a as a hard mask, so that a stack of theoxide semiconductor layers 130 a and 130 b and the conductive layer 141a is formed (see FIG. 44B). Here, oxygen ions may be implanted into theoxide semiconductor layers 130 a and 130 b.

Then, a second resist mask is formed over portions to be a source regionand a drain region, and part of the conductive layer 141 a is etchedusing the second resist mask, so that the conductive layers 141 and 151are formed (see FIG. 44C).

After that, the oxide semiconductor film 130C to be the oxidesemiconductor layer 130 c is formed over the stack of the oxidesemiconductor layers 130 a and 130 b and the conductive layers 141 and151. Furthermore, the insulating film 160A, the third conductive film171A, and the fourth conductive film 172A are formed over the oxidesemiconductor film 130C.

Then, the third resist mask 156 is formed over the fourth conductivefilm 172A (see FIG. 45A). The third conductive film 171A, the fourthconductive film 172A, the insulating film 160A, and the oxidesemiconductor film 130C are selectively etched using the third resistmask 156, so that the conductive layer 170 including the conductivelayers 171 and 172, the insulating layer 160, and the oxidesemiconductor layer 130 c are formed (see FIG. 45B).

Next, the insulating layers 175 and 180 are formed over the insulatinglayer 120, the oxide semiconductor layer 130 (the oxide semiconductorlayers 130 a to 130 c), the conductive layers 140 and 150, theinsulating layer 160, and the conductive layer 170.

Next, openings reaching the conductive layers 141 and 151 are providedin the insulating layers 175 and 180, and a fifth conductive film isformed to cover the openings. Then, a fourth resist mask is providedover the fifth conductive film and the fifth conductive film isselectively etched using the resist mask, so that the conductive layers142 and 152 are formed (see FIG. 45C).

Through the above steps, the transistor 111 can be manufactured.

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films that are described in thisembodiment typically can be formed by sputtering or plasma-enhanced CVD,such films may be formed by another method such as thermal CVD. Examplesof thermal CVD include metal organic chemical vapor deposition (MOCVD)and atomic layer deposition (ALD).

Since plasma is not used for deposition, thermal CVD has an advantagethat no defect due to plasma damage is generated.

Deposition by thermal CVD may be performed in such a manner that asource gas and an oxidizer are supplied to the chamber at the same time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

Deposition by ALD is performed in such a manner that the pressure in achamber is set to an atmospheric pressure or a reduced pressure, sourcegases for reaction are introduced into the chamber and reacted, and thenthe sequence of gas introduction is repeated. An inert gas (e.g., argonor nitrogen) may be introduced as a carrier gas with the source gases.For example, two or more kinds of source gases may be sequentiallysupplied to the chamber. In that case, after reaction of a first sourcegas, an inert gas is introduced, and then a second source gas isintroduced so that the source gases are not mixed. Alternatively, thefirst source gas may be exhausted by vacuum evacuation instead ofintroduction of the inert gas, and then the second source gas may beintroduced. The first source gas is adsorbed on the surface of thesubstrate and reacted to form a first layer, and then, the second sourcegas introduced is absorbed and reacted. As a result, a second layer isstacked over the first layer, so that a thin film is formed. Thesequence of gas introduction is controlled and repeated more than onceuntil desired thickness is obtained, so that a thin film with excellentstep coverage can be formed. The thickness of the thin film can beadjusted by the number of repetition times of the sequence of gasintroduction; therefore, ALD makes it possible to accurately adjustthickness and thus is suitable for manufacturing a minute FET.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film that have been disclosed in theembodiments can be formed by thermal CVD such as MOCVD or ALD. Forexample, in the case where an In—Ga—Zn—O film is formed, trimethylindium(In(CH₃)₃), trimethylgallium (Ga(CH₃)₃), and dimethylzinc (Zn(CH₃)₂) canbe used. Without limitation to the above combination, triethylgallium(Ga(C₂H₅)₃) can be used instead of trimethylgallium and diethylzinc(Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by adeposition apparatus using ALD, two kinds of gases, i.e., ozone (O₃) asan oxidizer and a source material gas which is obtained by vaporizingliquid containing a solvent and a hafnium precursor (hafnium alkoxideand a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH,Hf[N(CH₃)₂]₄) and tetrakis(ethylmethylamide)hafnium) are used.

For example, in the case where an aluminum oxide film is formed by adeposition apparatus using ALD, two kinds of gases, i.e., H₂O as anoxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor (e.g., trimethylaluminum(TMA, Al(CH₃)₃)) are used. Examples of another material includetris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, and radicals of an oxidizing gas(e.g., O₂ or dinitrogen monoxide) are supplied to react with anadsorbate.

For example, in the case where a tungsten film is formed by a depositionapparatus using ALD, a WF₆ gas and a B₂H₆ gas are sequentiallyintroduced to form an initial tungsten film, and then a WF₆ gas and anH₂ gas are sequentially introduced to form a tungsten film. Note that anSiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed by a deposition apparatus using ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced to form an In—Olayer, a Ga(CH₃)₃ gas and an O₃ gas are sequentially introduced to forma Ga-0 layer, and then a Zn(CH₃)₂ gas and an O₃ gas are sequentiallyintroduced to form a Zn—O layer. Note that the order of these layers isnot limited to this example. A mixed compound layer such as an In—Ga—Olayer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by using thesegases. Although an H₂O gas which is obtained by bubbling with an inertgas such as Ar may be used instead of an O₃ gas, it is preferable to usean O₃ gas, which does not contain H.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 7

The structure of an oxide semiconductor film that can be used for oneembodiment of the present invention is described below.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Theterm “perpendicular” indicates that an angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly includes the case where the angle is greater than orequal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

An oxide semiconductor film is roughly classified into anon-single-crystal oxide semiconductor film and a single-crystal oxidesemiconductor film. The non-single-crystal oxide semiconductor filmmeans any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS)film, a polycrystalline oxide semiconductor film, a microcrystallineoxide semiconductor film, an amorphous oxide semiconductor film, and thelike.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.Consequently, a plurality of crystal parts are observed clearly.However, in the high-resolution TEM image, a boundary between crystalparts, i.e., a grain boundary is not observed clearly. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface (cross-sectional TEM image), metal atoms are arranged in alayered manner in the crystal parts. Each metal atom layer has amorphology that reflects a surface over which the CAAC-OS film is formed(also referred to as a formation surface) or a top surface of theCAAC-OS film, and is provided parallel to the formation surface or thetop surface of the CAAC-OS film.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

The CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic order of theoxide semiconductor film by depriving the oxide semiconductor film ofoxygen and causes a decrease in crystallinity. Furthermore, a heavymetal such as iron or nickel, argon, carbon dioxide, or the like has alarge atomic radius (molecular radius), and thus disturbs the atomicorder of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas “highly purified intrinsic” or “substantially highly purifiedintrinsic.” A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film has few carrier generation sources,and thus can have low carrier density. Thus, a transistor including theoxide semiconductor film rarely has negative threshold voltage (israrely normally on). The highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has few carriertraps. Accordingly, the transistor including the oxide semiconductorfilm has few variations in electrical characteristics and highreliability. Charge trapped by the carrier traps in the oxidesemiconductor film takes a long time to be released and may behave likefixed charge. Thus, the transistor that includes the oxide semiconductorfilm having high impurity concentration and high density of defectstates has unstable electrical characteristics in some cases.

In a transistor including the CAAC-OS film, changes in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light are small.

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high-resolution TEM image and a region where acrystal part is not clearly observed in a high-resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductorfilm is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as ananocrystalline oxide semiconductor (nc-OS) film. In a high-resolutionTEM image, a crystal grain boundary cannot be found clearly in the nc-OSfilm in some cases.

In the nc-OS film, a microscopic region (e.g., a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has periodic atomic order. There is no regularityof crystal orientation between different crystal parts in the nc-OSfilm. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak that shows a crystalplane does not appear. Furthermore, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter larger than the diameterof a crystal part (e.g., larger than or equal to 50 nm). Meanwhile,spots are shown in a nanobeam electron diffraction pattern of the nc-OSfilm obtained by using an electron beam having a probe diameter close toor smaller than the diameter of a crystal part. Furthermore, in ananobeam electron diffraction pattern of the nc-OS film,circumferentially distributed spots are observed in some cases. Also ina nanobeam electron diffraction pattern of the nc-OS film, a pluralityof spots are shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has higher regularitythan an amorphous oxide semiconductor film. Thus, the nc-OS film has alower density of defect states than the amorphous oxide semiconductorfilm. Note that there is no regularity of crystal orientation betweendifferent crystal parts in the nc-OS film; thus, the nc-OS film has ahigher density of defect states than the CAAC-OS film.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In a high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Furthermore, a halo pattern is shown but a spot is not shown in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beseen.

Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed. In the a-like OS film, crystallization by a slight amountof electron beam used for TEM observation occurs and growth of thecrystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is less observedin the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers of three In—O layers and six Ga—Zn—Olayers are layered in the c-axis direction. Accordingly, the spacingbetween these adjacent layers is equivalent to the lattice spacing onthe (009) plane (also referred to as a d value). The value is calculatedto be 0.29 nm from crystal structure analysis. Thus, each of the latticefringes in which the spacing therebetween is from 0.28 nm to 0.30 nmcorresponds to the a-b plane of the InGaZnO₄ crystal, focusing on thelattice fringes in the high-resolution TEM image.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, an a-like OSfilm, a microcrystalline oxide semiconductor film, and a CAAC-OS film,for example.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 8

An imaging device in one embodiment of the present invention and asemiconductor device including the imaging device can be used fordisplay devices, personal computers, or image reproducing devicesprovided with recording media (typically, devices that reproduce thecontent of recording media such as digital versatile discs (DVD) andhave displays for displaying the reproduced images). Furthermore, aselectronic devices that can include the imaging device in one embodimentof the present invention and the semiconductor device including theimaging device, cellular phones, game machines (including portable gamemachines), portable information terminals, e-book readers, cameras suchas video cameras and digital still cameras, goggle-type displays (headmounted displays), navigation systems, audio reproducing devices (e.g.,car audio systems and digital audio players), copiers, facsimiles,printers, multifunction printers, automated teller machines (ATM),vending machines, and the like can be given. FIGS. 46A to 46F illustratespecific examples of these electronic devices.

FIG. 46A illustrates a portable game machine, which includes housings901 and 902, display portions 903 and 904, a microphone 905, speakers906, an operation key 907, a stylus 908, a camera 909, and the like.Although the portable game machine in FIG. 46A has the two displayportions 903 and 904, the number of display portions included in theportable game machine is not limited to this. The imaging device in oneembodiment of the present invention can be used for the camera 909.

FIG. 46B illustrates a portable data terminal, which includes a housing911, a display portion 912, a camera 919, and the like. The touch panelfunction of the display portion 912 enables input and output ofinformation. The imaging device in one embodiment of the presentinvention can be used for the camera 919.

FIG. 46C illustrates a digital camera, which includes a housing 921, ashutter button 922, a microphone 923, a light-emitting portion 927, alens 925, and the like. The imaging device in one embodiment of thepresent invention can be provided in a focus position of the lens 925.

FIG. 46D illustrates a wrist-watch-type information terminal, whichincludes a housing 931, a display portion 932, a wristband 933, a camera939, and the like. The display portion 932 may be a touch panel. Theimaging device in one embodiment of the present invention can be usedfor the camera 939.

FIG. 46E illustrates a video camera, which includes a first housing 941,a second housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andan angle between the first housing 941 and the second housing 942 can bechanged with the joint 946. An image displayed on the display portion943 may be switched in accordance with the angle between the firsthousing 941 and the second housing 942 at the joint 946. The imagingdevice in one embodiment of the present invention can be provided in afocus position of the lens 945.

FIG. 46F illustrates a copier, which includes a housing 951 providedwith a reading portion 952, an operation portion 953, a sensor 954, andthe like. The imaging device in one embodiment of the present inventionin which pixels are arranged in a line can be used for the sensor 954,and data in a plane can be read by scanning in a direction perpendicularto the pixel arrangement. The same structure can also be applied to afacsimile or a scanner.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

This application is based on Japanese Patent Application serial No.2014-227707 filed with Japan Patent Office on Nov. 10, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An imaging device comprising: a first circuit;and a second circuit, wherein the first circuit includes a photoelectricconversion element, a first transistor, a second transistor, a thirdtransistor, a fourth transistor, a fifth transistor, a sixth transistor,a seventh transistor, a first capacitor, a second capacitor, and a thirdcapacitor, wherein the second circuit includes an eighth transistor,wherein one terminal of the photoelectric conversion element iselectrically connected to one of a source and a drain of the firsttransistor, wherein the other of the source and the drain of the firsttransistor is electrically connected to one of a source and a drain ofthe second transistor, wherein the other of the source and the drain ofthe first transistor is electrically connected to one terminal of thefirst capacitor, wherein one of a source and a drain of the thirdtransistor is electrically connected to the other terminal of the firstcapacitor; wherein the other terminal of the first capacitor iselectrically connected to one terminal of the second capacitor, whereinone of a source and a drain of the fourth transistor is electricallyconnected to the other terminal of the second capacitor, wherein theother of the source and the drain of the fourth transistor iselectrically connected to one of a source and a drain of the fifthtransistor, wherein one of a source and a drain of the sixth transistoris electrically connected to one of a source and a drain of the fifthtransistor, wherein one terminal of the third capacitor is electricallyconnected to the other terminal of the second capacitor, wherein theother terminal of the third capacitor is electrically connected to theother of the source and the drain of the sixth transistor, wherein agate of the sixth transistor is electrically connected to the oneterminal of the third capacitor, wherein one of a source and a drain ofthe seventh transistor is electrically connected to the other of thesource and the drain of the sixth transistor, wherein the other of thesource and the drain of the seventh transistor is electrically connectedto one of a source and a drain of the eighth transistor, and wherein thephotoelectric conversion element contains selenium in a photoelectricconversion layer.
 2. The imaging device according to claim 1, whereinthe second circuit includes a ninth transistor, wherein one of a sourceand a drain of the ninth transistor is electrically connected to theother of the source and the drain of the eighth transistor, wherein agate of the ninth transistor is electrically connected to a gate of theeighth transistor, and wherein the gate of the ninth transistor iselectrically connected to the other of the source and the drain of theninth transistor.
 3. The imaging device according to claim 1, whereinthe other of the source and the drain of the third transistor iselectrically connected to the other terminal of the photoelectricconversion element.
 4. The imaging device according to claim 1, whereinthe pixel circuit includes a fourth capacitor, and wherein one terminalof the fourth capacitor is electrically connected to the one of thesource and the drain of the third transistor.
 5. The imaging deviceaccording to claim 4, wherein the other terminal of the fourth capacitoris electrically connected to the other of the source and the drain ofthe fifth transistor.
 6. The imaging device according to claim 1,wherein one or all of the first to ninth transistors include oxidesemiconductors in active layers, and wherein the oxide semiconductorcontains In, Zn, and M (M is Al, Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf).7. An electronic device comprising a display device, an operation key, ashutter button, and the imaging device according to claim 1.